diff --git a/elasticgraph-protobuf/README.md b/elasticgraph-protobuf/README.md
index 3d32c16c4..6465f796e 100644
--- a/elasticgraph-protobuf/README.md
+++ b/elasticgraph-protobuf/README.md
@@ -162,6 +162,36 @@ end
 When a mapping exists for an enum, `elasticgraph-protobuf` uses the mapped proto enum(s)
 as the source of enum values (respecting `exclusions`, `expected_extras`, and `name_transform`).
 
+### Referencing Existing Protobuf Types
+
+For enums that exactly match a canonical proto enum, you can import and reference
+the existing proto type instead of generating a duplicate local enum:
+
+```ruby
+# in config/schema/protobuf.rb
+
+ElasticGraph.define_schema do |schema|
+  if defined?(Squareup::Connect::V2::Resources::Card::Type)
+    schema.proto_enum_mappings(
+      "CardType" => {
+        Squareup::Connect::V2::Resources::Card::Type => {}
+      }
+    )
+
+    schema.proto_external_types(
+      "CardType" => {
+        proto: "squareup.connect.v2.resources.Card.Type",
+        import: "squareup/connect/v2/resources/card.proto"
+      }
+    )
+  end
+end
+```
+
+External type references currently support enums only. The matching
+`proto_enum_mappings` entry must have exactly one source and no transform options;
+otherwise the enum stays generated locally so value curation remains explicit.
+
 ### Stable Field Numbers
 
 `schema_artifacts:dump` automatically reads and writes `proto_field_numbers.yaml`
diff --git a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/api_extension.rb b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/api_extension.rb
index 76cac0902..b61bdf635 100644
--- a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/api_extension.rb
+++ b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/api_extension.rb
@@ -106,6 +106,26 @@ def proto_enum_mappings(proto_enums_by_graphql_enum)
           nil
         end
 
+        # Registers GraphQL types that should be referenced from existing proto files instead of
+        # generated locally in `schema.proto`.
+        #
+        # @param proto_external_types [Hash] map of GraphQL type name to `proto` and `import` values
+        # @return [void]
+        #
+        # @example Reference an external enum type
+        #   ElasticGraph.define_schema do |schema|
+        #     schema.proto_external_types(
+        #       "CardType" => {
+        #         proto: "squareup.connect.v2.resources.Card.Type",
+        #         import: "squareup/connect/v2/resources/card.proto"
+        #       }
+        #     )
+        #   end
+        def proto_external_types(proto_external_types)
+          protobuf_state.proto_external_types = proto_external_types
+          nil
+        end
+
         # Configures proto field-number mappings directly from a hash.
         # Useful for tests and advanced use cases where mappings are sourced outside artifacts.
         # When artifacts are dumped, mappings from the existing `proto_field_numbers.yaml` artifact
diff --git a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/identifier.rb b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/identifier.rb
index 28d4e3d59..c50f58134 100644
--- a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/identifier.rb
+++ b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/identifier.rb
@@ -51,6 +51,15 @@ def self.enum_value_name(name)
           escape_keyword(name.to_s)
         end
 
+        # Builds a reference to an externally-defined protobuf type. Unlike generated local
+        # identifiers, this preserves dotted fully-qualified names verbatim.
+        #
+        # @param name [#to_s]
+        # @return [String]
+        def self.external_type_name(name)
+          name.to_s
+        end
+
         # Escapes protobuf reserved keywords by suffixing them with an underscore.
         #
         # @param identifier [String]
diff --git a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/results_extension.rb b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/results_extension.rb
index 43281f78b..57aecc12a 100644
--- a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/results_extension.rb
+++ b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/results_extension.rb
@@ -52,6 +52,7 @@ def protobuf_schema_generator
               state: state,
               package_name: state.proto_schema_package_name,
               proto_enums_by_graphql_enum: state.proto_enums_by_graphql_enum,
+              proto_external_types: state.proto_external_types,
               proto_field_number_mappings: state.proto_field_number_mappings,
               syntax: state.proto_schema_syntax,
               headers: state.proto_schema_headers
diff --git a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/schema.rb b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/schema.rb
index 50c41bd59..79a106da8 100644
--- a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/schema.rb
+++ b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/schema.rb
@@ -60,6 +60,13 @@ class Schema
         # @!attribute [r] name_in_index
         #   @return [String]
         FieldNumberMapping = ::Data.define(:field_number, :name_in_index)
+        # Internal representation of an externally-defined protobuf type.
+        #
+        # @!attribute [r] fqn
+        #   @return [String]
+        # @!attribute [r] import
+        #   @return [String]
+        ExternalTypeDefinition = ::Data.define(:fqn, :import)
 
         # Protobuf syntaxes this generator can emit.
         SUPPORTED_SYNTAXES = %w[proto2 proto3].freeze
@@ -67,6 +74,7 @@ class Schema
         # @param state [ElasticGraph::SchemaDefinition::State]
         # @param package_name [String]
         # @param proto_enums_by_graphql_enum [Hash]
+        # @param proto_external_types [Hash]
         # @param proto_field_number_mappings [Hash]
         # @param syntax [Symbol, String] `:proto3` (default) or `:proto2`; validated by {APIExtension#proto_schema_artifacts}
         # @param headers [Array<String>] file-level header lines (e.g. `option` declarations) rendered verbatim
@@ -74,6 +82,7 @@ def initialize(
           state:,
           package_name:,
           proto_enums_by_graphql_enum:,
+          proto_external_types: {},
           proto_field_number_mappings: {},
           syntax: :proto3,
           headers: []
@@ -83,7 +92,10 @@ def initialize(
           @state = state
           @package_name = Identifier.package_name(package_name)
           @proto_enums_by_graphql_enum = normalize_proto_enum_mappings(proto_enums_by_graphql_enum)
+          @proto_external_types_by_type_name = normalize_proto_external_types(proto_external_types)
           @proto_field_number_mappings_by_message = normalize_proto_field_number_mappings(proto_field_number_mappings)
+          @imports = ::Set.new
+          @registered_external_type_names = ::Set.new
           @message_definitions_by_name = {}
           @enum_definitions_by_name = {}
           @generated_message_definitions_by_name = {}
@@ -105,8 +117,9 @@ def to_proto
             %(syntax = "#{@syntax}";),
             "package #{@package_name};",
             *render_headers,
+            *render_imports,
             render_definitions
-          ]
+          ].reject(&:empty?)
 
           sections.join("\n\n") + "\n"
         end
@@ -146,6 +159,11 @@ def indexed_types
 
         # Registers the type's proto definition (if it needs one) and returns its proto field type name.
         def register_type(type)
+          if type.respond_to?(:name) && (external_type = @proto_external_types_by_type_name[type.name.to_s])
+            register_external_type(type, external_type)
+            return external_type.fqn
+          end
+
           case type
           when SchemaElements::EnumTypeExtension
             register_enum(type)
@@ -161,6 +179,55 @@ def register_type(type)
           type.to_proto_field_type
         end
 
+        def register_external_type(type, external_type)
+          type_name = type.name.to_s
+
+          case type
+          when SchemaElements::EnumTypeExtension
+            unless @registered_external_type_names.include?(type_name)
+              validate_external_enum_type(type)
+              @registered_external_type_names << type_name
+            end
+
+            @imports << external_type.import
+          else
+            raise Errors::SchemaError, "External proto type `#{type.name}` cannot be referenced yet. " \
+              "Only enum types are supported by `proto_external_types` in this release."
+          end
+        end
+
+        def validate_external_enum_type(enum_type)
+          enum_type_name = enum_type.name.to_s
+          mapping_entries = @proto_enums_by_graphql_enum[enum_type_name]
+          if mapping_entries.nil? || mapping_entries.empty?
+            raise Errors::SchemaError, "External proto enum `#{enum_type_name}` must also configure " \
+              "`proto_enum_mappings` with exactly one untransformed source so its values can be verified."
+          end
+
+          unless mapping_entries.size == 1
+            raise Errors::SchemaError, "External proto enum `#{enum_type_name}` must use exactly one " \
+              "`proto_enum_mappings` source; multi-source enum mappings cannot be safely referenced externally."
+          end
+
+          proto_type, options = mapping_entries.first
+          options_are_empty = options.nil? || (options.is_a?(Hash) && options.empty?)
+          unless options_are_empty
+            raise Errors::SchemaError, "External proto enum `#{enum_type_name}` must use an empty " \
+              "`proto_enum_mappings` options hash; transformed, excluded, or extra values must stay generated locally."
+          end
+
+          proto_value_names = enum_value_names_from_proto_mapping(
+            enum_type_name: enum_type_name,
+            proto_type: proto_type,
+            options: {}
+          ).uniq.sort
+          eg_value_names = enum_type.values_by_name.keys.map(&:to_s).uniq.sort
+          return if proto_value_names == eg_value_names
+
+          raise Errors::SchemaError, "External proto enum `#{enum_type_name}` values do not match the ElasticGraph enum values. " \
+            "External values: #{proto_value_names.join(", ")}. ElasticGraph values: #{eg_value_names.join(", ")}."
+        end
+
         def register_message(type)
           message_name = Identifier.message_name(type.name)
           check_message_name_collision(message_name, type.name)
@@ -428,6 +495,10 @@ def name_taken?(name)
             @enum_definitions_by_name.key?(name)
         end
 
+        def render_imports
+          @imports.sort.map { |import| "import \"#{import}\";" }
+        end
+
         # Renders the custom header lines as a single contiguous section (so they are not
         # blank-line separated). Returns `[]` when no headers were configured.
         def render_headers
@@ -543,6 +614,46 @@ def normalize_proto_enum_mappings(raw_mappings)
           normalized
         end
 
+        def normalize_proto_external_types(raw_mappings)
+          normalized = {} # : ::Hash[::String, ExternalTypeDefinition]
+          return normalized if raw_mappings.nil?
+
+          unless raw_mappings.is_a?(Hash)
+            raise Errors::SchemaError, "External proto type mappings must be a Hash, got: #{raw_mappings.class}."
+          end
+
+          raw_mappings.each do |type_name, mapping|
+            unless mapping.is_a?(Hash)
+              raise Errors::SchemaError, "External proto type mapping for `#{type_name}` must be a Hash."
+            end
+
+            proto_type_name = fetch_external_type_mapping_value(type_name, mapping, :proto)
+            import = fetch_external_type_mapping_value(type_name, mapping, :import)
+
+            normalized[type_name.to_s] = ExternalTypeDefinition.new(
+              fqn: Identifier.external_type_name(proto_type_name),
+              import: import
+            )
+          end
+
+          normalized
+        end
+
+        def fetch_external_type_mapping_value(type_name, mapping, key)
+          value =
+            if mapping.key?(key)
+              mapping.fetch(key)
+            elsif mapping.key?(key.to_s)
+              mapping.fetch(key.to_s)
+            end
+
+          if value.is_a?(String) && !value.empty?
+            value
+          else
+            raise Errors::SchemaError, "External proto type mapping for `#{type_name}` must include a non-empty `#{key}` String."
+          end
+        end
+
         def normalize_proto_field_number_mappings(raw_mappings)
           return {} if raw_mappings.nil?
           unless raw_mappings.is_a?(Hash)
diff --git a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/state_extension.rb b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/state_extension.rb
index 96f3e3fc0..3eb207b3b 100644
--- a/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/state_extension.rb
+++ b/elasticgraph-protobuf/lib/elastic_graph/protobuf/schema_definition/state_extension.rb
@@ -15,15 +15,17 @@ module SchemaDefinition
       module StateExtension
         # @dynamic proto_schema_package_name, proto_schema_package_name=
         # @dynamic proto_enums_by_graphql_enum, proto_enums_by_graphql_enum=
+        # @dynamic proto_external_types, proto_external_types=
         # @dynamic proto_field_number_mappings, proto_field_number_mappings=
         # @dynamic proto_schema_syntax, proto_schema_syntax=
         # @dynamic proto_schema_headers, proto_schema_headers=
-        attr_accessor :proto_schema_package_name, :proto_enums_by_graphql_enum,
+        attr_accessor :proto_schema_package_name, :proto_enums_by_graphql_enum, :proto_external_types,
           :proto_field_number_mappings, :proto_schema_syntax, :proto_schema_headers
 
         def self.extended(state)
           state.proto_schema_package_name = "elasticgraph"
           state.proto_enums_by_graphql_enum = {}
+          state.proto_external_types = {}
           state.proto_field_number_mappings = {}
           state.proto_schema_syntax = :proto3
           state.proto_schema_headers = []
diff --git a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/api_extension.rbs b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/api_extension.rbs
index 77fc192df..71b1d12aa 100644
--- a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/api_extension.rbs
+++ b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/api_extension.rbs
@@ -7,6 +7,7 @@ module ElasticGraph
         def self.extended: (::ElasticGraph::SchemaDefinition::API & APIExtension) -> void
         def proto_schema_artifacts: (?package_name: ::String, ?syntax: ::Symbol, ?headers: ::Array[::String]) -> void
         def proto_enum_mappings: (untyped) -> void
+        def proto_external_types: (untyped) -> void
         def configure_proto_field_number_mappings: (untyped) -> void
 
         private
diff --git a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/identifier.rbs b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/identifier.rbs
index 0a19bd32d..c8eeedbcb 100644
--- a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/identifier.rbs
+++ b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/identifier.rbs
@@ -9,6 +9,7 @@ module ElasticGraph
         def self.enum_name: (::String) -> ::String
         def self.field_name: (::String) -> ::String
         def self.enum_value_name: (::String) -> ::String
+        def self.external_type_name: (::String) -> ::String
         def self.escape_keyword: (::String) -> ::String
       end
     end
diff --git a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/schema.rbs b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/schema.rbs
index ffd4a7ed4..764509e7c 100644
--- a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/schema.rbs
+++ b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/schema.rbs
@@ -51,6 +51,13 @@ module ElasticGraph
           def self.new: (field_number: ::Integer, name_in_index: ::String) -> instance
         end
 
+        class ExternalTypeDefinition
+          attr_reader fqn: ::String
+          attr_reader import: ::String
+
+          def self.new: (fqn: ::String, import: ::String) -> instance
+        end
+
         type fieldNumberMappingsByFieldName = ::Hash[::String, FieldNumberMapping]
 
         SUPPORTED_SYNTAXES: ::Array[::String]
@@ -60,7 +67,10 @@ module ElasticGraph
         @state: ::ElasticGraph::SchemaDefinition::State
         @package_name: ::String
         @proto_enums_by_graphql_enum: ::Hash[::String, untyped]
+        @proto_external_types_by_type_name: ::Hash[::String, ExternalTypeDefinition]
         @proto_field_number_mappings_by_message: ::Hash[::String, fieldNumberMappingsByFieldName]
+        @imports: ::Set[::String]
+        @registered_external_type_names: ::Set[::String]
         @message_definitions_by_name: ::Hash[::String, MessageDefinition]
         @enum_definitions_by_name: ::Hash[::String, EnumDefinition]
         @generated_message_definitions_by_name: ::Hash[::String, MessageDefinition]
@@ -73,6 +83,7 @@ module ElasticGraph
           state: ::ElasticGraph::SchemaDefinition::State,
           package_name: ::String,
           proto_enums_by_graphql_enum: untyped,
+          ?proto_external_types: untyped,
           ?proto_field_number_mappings: untyped,
           ?syntax: (::Symbol | ::String),
           ?headers: ::Array[::String]
@@ -85,6 +96,8 @@ module ElasticGraph
 
         def indexed_types: () -> ::Array[untyped]
         def register_type: (untyped type) -> ::String
+        def register_external_type: (untyped type, ExternalTypeDefinition) -> void
+        def validate_external_enum_type: (untyped enum_type) -> void
         def register_message: (untyped type) -> void
         def register_enum: (untyped enum_type) -> void
         def enum_value_names_for: (untyped enum_type) -> ::Array[::String]
@@ -121,6 +134,7 @@ module ElasticGraph
         ) -> ::String
         def unique_generated_message_name: (::String) -> ::String
         def name_taken?: (::String) -> bool
+        def render_imports: () -> ::Array[::String]
         def render_headers: () -> ::Array[::String]
         def render_definitions: () -> ::String
         def proto_enum_value_name: (::String, ::String) -> ::String
@@ -135,6 +149,8 @@ module ElasticGraph
         def check_message_name_collision: (::String, ::String) -> void
         def check_enum_name_collision: (::String, ::String) -> void
         def normalize_proto_enum_mappings: (untyped) -> ::Hash[::String, untyped]
+        def normalize_proto_external_types: (untyped) -> ::Hash[::String, ExternalTypeDefinition]
+        def fetch_external_type_mapping_value: (::String | ::Symbol, ::Hash[untyped, untyped], ::Symbol) -> ::String
         def normalize_proto_field_number_mappings: (untyped) -> ::Hash[::String, fieldNumberMappingsByFieldName]
         def normalize_field_number_mapping_entry: (::String, ::String, untyped) -> [::Integer, ::String]
         def renamed_public_field_names_by_type_name: () -> ::Hash[::String, ::Hash[::String, ::Array[::String]]]
diff --git a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/state_extension.rbs b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/state_extension.rbs
index a1349ed46..29642503f 100644
--- a/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/state_extension.rbs
+++ b/elasticgraph-protobuf/sig/elastic_graph/protobuf/schema_definition/state_extension.rbs
@@ -4,6 +4,7 @@ module ElasticGraph
       module StateExtension: ::ElasticGraph::SchemaDefinition::State
         attr_accessor proto_schema_package_name: ::String
         attr_accessor proto_enums_by_graphql_enum: untyped
+        attr_accessor proto_external_types: untyped
         attr_accessor proto_field_number_mappings: untyped
         attr_accessor proto_schema_syntax: (::Symbol | ::String)
         attr_accessor proto_schema_headers: ::Array[::String]
diff --git a/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/identifier_spec.rb b/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/identifier_spec.rb
index fb87afe97..e1381fad2 100644
--- a/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/identifier_spec.rb
+++ b/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/identifier_spec.rb
@@ -27,6 +27,11 @@ module SchemaDefinition
           expect(Identifier.field_name("string")).to eq("string_")
           expect(Identifier.enum_value_name("stream")).to eq("stream_")
         end
+
+        it "preserves external dotted type names" do
+          expect(Identifier.external_type_name("squareup.connect.v2.resources.Card.Type"))
+            .to eq("squareup.connect.v2.resources.Card.Type")
+        end
       end
     end
   end
diff --git a/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_edge_cases_spec.rb b/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_edge_cases_spec.rb
index a6d434a2e..97e9ac7d5 100644
--- a/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_edge_cases_spec.rb
+++ b/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_edge_cases_spec.rb
@@ -163,6 +163,228 @@ def self.enums
           expect(generated).to include("STATUS_LEGACY = 2;")
         end
 
+        it "requires external enum types to have exactly one enum mapping source" do
+          results = define_proto_schema do |s|
+            s.proto_external_types(
+              "Status" => {
+                proto: "squareup.connect.v2.Status",
+                import: "squareup/connect/v2/status.proto"
+              }
+            )
+
+            s.enum_type "Status" do |t|
+              t.values "ACTIVE"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "status", "Status"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("must also configure `proto_enum_mappings`"))
+        end
+
+        it "rejects external enum types with transformed enum mappings" do
+          results = define_proto_schema do |s|
+            s.proto_enum_mappings(
+              "Status" => {
+                ::Object.new => {
+                  expected_extras: [:LEGACY]
+                }
+              }
+            )
+            s.proto_external_types(
+              "Status" => {
+                proto: "squareup.connect.v2.Status",
+                import: "squareup/connect/v2/status.proto"
+              }
+            )
+
+            s.enum_type "Status" do |t|
+              t.values "ACTIVE"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "status", "Status"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("must use an empty `proto_enum_mappings` options hash"))
+        end
+
+        it "rejects external enum types with multiple enum mapping sources" do
+          results = define_proto_schema do |s|
+            s.proto_enum_mappings("Status" => {::Object.new => {}, ::Object.new => {}})
+            s.proto_external_types(
+              "Status" => {
+                proto: "squareup.connect.v2.Status",
+                import: "squareup/connect/v2/status.proto"
+              }
+            )
+
+            s.enum_type "Status" do |t|
+              t.values "ACTIVE"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "status", "Status"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("must use exactly one `proto_enum_mappings` source"))
+        end
+
+        it "deduplicates imports for repeated external enum references" do
+          proto_status = ::Class.new do
+            def self.enums
+              [::Data.define(:name).new(name: :ACTIVE)]
+            end
+          end
+
+          results = define_proto_schema do |s|
+            s.proto_enum_mappings("Status" => {proto_status => {}})
+            s.proto_external_types(
+              "Status" => {
+                "proto" => "squareup.connect.v2.Status",
+                "import" => "squareup/connect/v2/status.proto"
+              }
+            )
+
+            s.enum_type "Status" do |t|
+              t.values "ACTIVE"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "status", "Status"
+              t.field "previous_status", "Status"
+              t.index "accounts"
+            end
+          end
+
+          generated = proto_schema_from(results)
+          expect(generated.scan('import "squareup/connect/v2/status.proto";').size).to eq(1)
+          expect(generated).to include("squareup.connect.v2.Status status = 2;")
+          expect(generated).to include("squareup.connect.v2.Status previous_status = 3;")
+        end
+
+        it "validates external enum values against the ElasticGraph enum values" do
+          proto_status = ::Class.new do
+            def self.enums
+              [
+                ::Data.define(:name).new(name: :ACTIVE),
+                ::Data.define(:name).new(name: :PENDING)
+              ]
+            end
+          end
+
+          results = define_proto_schema do |s|
+            s.proto_enum_mappings("Status" => {proto_status => {}})
+            s.proto_external_types(
+              "Status" => {
+                proto: "squareup.connect.v2.Status",
+                import: "squareup/connect/v2/status.proto"
+              }
+            )
+
+            s.enum_type "Status" do |t|
+              t.values "ACTIVE", "INACTIVE"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "status", "Status"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("values do not match"))
+        end
+
+        it "rejects external proto type references for non-enum types" do
+          results = define_proto_schema do |s|
+            s.proto_external_types(
+              "Address" => {
+                proto: "squareup.connect.v2.Address",
+                import: "squareup/connect/v2/address.proto"
+              }
+            )
+
+            s.object_type "Address" do |t|
+              t.field "street", "String"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "address", "Address"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("Only enum types are supported"))
+        end
+
+        it "validates external proto type mapping input type" do
+          results = define_proto_schema do |s|
+            s.proto_external_types("bad")
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("External proto type mappings must be a Hash"))
+        end
+
+        it "validates per-type external proto type mapping structure" do
+          results = define_proto_schema do |s|
+            s.proto_external_types("Status" => "bad")
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("External proto type mapping for `Status` must be a Hash"))
+        end
+
+        it "requires external proto type mappings to define proto and import strings" do
+          results = define_proto_schema do |s|
+            s.proto_external_types("Status" => {proto: "squareup.connect.v2.Status"})
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.index "accounts"
+            end
+          end
+
+          expect {
+            proto_schema_from(results)
+          }.to raise_error(Errors::SchemaError, a_string_including("must include a non-empty `import` String"))
+        end
+
         it "raises on field-number mapping collisions for a message" do
           results = define_proto_schema do |s|
             s.configure_proto_field_number_mappings(
@@ -332,6 +554,7 @@ def self.enums
             state: build_fake_state,
             package_name: "elasticgraph",
             proto_enums_by_graphql_enum: nil,
+            proto_external_types: nil,
             proto_field_number_mappings: nil
           )
 
diff --git a/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_spec.rb b/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_spec.rb
index e812e68fb..80745d86a 100644
--- a/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_spec.rb
+++ b/elasticgraph-protobuf/spec/unit/elastic_graph/protobuf/schema_definition/schema_spec.rb
@@ -493,6 +493,50 @@ def self.enums
           expect(generated).not_to include("OBSOLETE")
         end
 
+        it "can import and reference an external proto enum type" do
+          proto_status = ::Class.new do
+            def self.enums
+              [
+                ::Data.define(:name).new(name: :ACTIVE),
+                ::Data.define(:name).new(name: :INACTIVE)
+              ]
+            end
+          end
+
+          results = define_proto_schema do |s|
+            s.proto_enum_mappings("Status" => {proto_status => {}})
+            s.proto_external_types(
+              "Status" => {
+                proto: "squareup.connect.v2.Status",
+                import: "squareup/connect/v2/status.proto"
+              }
+            )
+
+            s.enum_type "Status" do |t|
+              t.values "ACTIVE", "INACTIVE"
+            end
+
+            s.object_type "Account" do |t|
+              t.field "id", "ID"
+              t.field "status", "Status"
+              t.index "accounts"
+            end
+          end
+
+          expect(proto_schema_from(results)).to eq(<<~PROTO)
+            syntax = "proto3";
+
+            package elasticgraph;
+
+            import "squareup/connect/v2/status.proto";
+
+            message Account {
+              string id = 1;
+              squareup.connect.v2.Status status = 2;
+            }
+          PROTO
+        end
+
         it "raises when mapped proto enum sources produce inconsistent values" do
           proto_status_a = ::Class.new do
             def self.enums