diff --git a/bundler/lib/bundler/cli.rb b/bundler/lib/bundler/cli.rb
index 3395e7455684..9c164b09f4ee 100644
--- a/bundler/lib/bundler/cli.rb
+++ b/bundler/lib/bundler/cli.rb
@@ -10,6 +10,7 @@ class CLI < Thor
 
     AUTO_INSTALL_CMDS = %w[show binstubs outdated exec open console licenses clean].freeze
     PARSEABLE_COMMANDS = %w[check config help exec platform show version].freeze
+    EXTENSIONS = ["c"].freeze
 
     COMMAND_ALIASES = {
       "check" => "c",
@@ -22,6 +23,8 @@ class CLI < Thor
     }.freeze
 
     def self.start(*)
+      check_deprecated_ext_option(ARGV) if ARGV.include?("--ext")
+
       super
     ensure
       Bundler::SharedHelpers.print_major_deprecations!
@@ -576,7 +579,7 @@ def viz
     method_option :edit, :type => :string, :aliases => "-e", :required => false, :banner => "EDITOR",
                          :lazy_default => [ENV["BUNDLER_EDITOR"], ENV["VISUAL"], ENV["EDITOR"]].find {|e| !e.nil? && !e.empty? },
                          :desc => "Open generated gemspec in the specified editor (defaults to $EDITOR or $BUNDLER_EDITOR)"
-    method_option :ext, :type => :boolean, :default => false, :desc => "Generate the boilerplate for C extension code"
+    method_option :ext, :type => :string, :desc => "Generate the boilerplate for C extension code.", :enum => EXTENSIONS
     method_option :git, :type => :boolean, :default => true, :desc => "Initialize a git repo inside your library."
     method_option :mit, :type => :boolean, :desc => "Generate an MIT license file. Set a default with `bundle config set --global gem.mit true`."
     method_option :rubocop, :type => :boolean, :desc => "Add rubocop to the generated Rakefile and gemspec. Set a default with `bundle config set --global gem.rubocop true`."
@@ -755,6 +758,38 @@ def self.reformatted_help_args(args)
       end
     end
 
+    def self.check_deprecated_ext_option(arguments)
+      # when deprecated version of `--ext` is called
+      # print out deprecation warning and pretend `--ext=c` was provided
+      if deprecated_ext_value?(arguments)
+        SharedHelpers.major_deprecation 2, "Option `--ext` without explicit value is deprecated. Please pass value like `--ext=c` for C extension. Pretending `--ext=c` was used for now."
+        arguments[arguments.index("--ext")] = "--ext=c"
+      end
+    end
+
+    def self.deprecated_ext_value?(arguments)
+      index = arguments.index("--ext")
+      next_argument = arguments[index+1]
+
+      # it is ok when --ext is followed with valid extension value
+      # for example `bundle gem hello --ext c`
+      return false if EXTENSIONS.include?(next_argument)
+
+      # deprecated call when --ext is called with no value in last position
+      # for example `bundle gem hello_gem --ext`
+      return true if next_argument.nil?
+
+      # deprecated call when --ext is followed by other parameter
+      # for example `bundle gem --ext --no-ci hello_gem`
+      return true if next_argument.start_with?("-")
+
+      # deprecated call when --ext is followed by gem name
+      # for example `bundle gem --ext hello_gem`
+      return true if next_argument
+
+      false
+    end
+
     private
 
     # Automatically invoke `bundle install` and resume if
diff --git a/bundler/lib/bundler/cli/gem.rb b/bundler/lib/bundler/cli/gem.rb
index 135bfde03840..584b28d67d48 100644
--- a/bundler/lib/bundler/cli/gem.rb
+++ b/bundler/lib/bundler/cli/gem.rb
@@ -15,7 +15,7 @@ class CLI::Gem
       "test-unit" => "3.0",
     }.freeze
 
-    attr_reader :options, :gem_name, :thor, :name, :target
+    attr_reader :options, :gem_name, :thor, :name, :target, :extension
 
     def initialize(options, gem_name, thor)
       @options = options
@@ -28,7 +28,9 @@ def initialize(options, gem_name, thor)
       @name = @gem_name
       @target = SharedHelpers.pwd.join(gem_name)
 
-      validate_ext_name if options[:ext]
+      @extension = options[:ext]
+
+      validate_ext_name if @extension
     end
 
     def run
@@ -64,7 +66,7 @@ def run
         :author => git_author_name.empty? ? "TODO: Write your name" : git_author_name,
         :email => git_user_email.empty? ? "TODO: Write your email address" : git_user_email,
         :test => options[:test],
-        :ext => options[:ext],
+        :ext => extension,
         :exe => options[:exe],
         :bundler_version => bundler_dependency_version,
         :git => use_git,
@@ -188,7 +190,7 @@ def run
 
       templates.merge!("exe/newgem.tt" => "exe/#{name}") if config[:exe]
 
-      if options[:ext]
+      if extension
         templates.merge!(
           "ext/newgem/extconf.rb.tt" => "ext/#{name}/extconf.rb",
           "ext/newgem/newgem.h.tt" => "ext/#{name}/#{underscored_name}.h",
diff --git a/bundler/lib/bundler/man/bundle-gem.1 b/bundler/lib/bundler/man/bundle-gem.1
index 8a82dfdd2a00..e32a03daf409 100644
--- a/bundler/lib/bundler/man/bundle-gem.1
+++ b/bundler/lib/bundler/man/bundle-gem.1
@@ -48,7 +48,7 @@ Add a \fBCODE_OF_CONDUCT\.md\fR file to the root of the generated project\. If t
 Do not create a \fBCODE_OF_CONDUCT\.md\fR (overrides \fB\-\-coc\fR specified in the global config)\.
 .
 .TP
-\fB\-\-ext\fR
+\fB\-\-ext=c\fR
 Add boilerplate for C extension code to the generated project\. This behavior is disabled by default\.
 .
 .TP
diff --git a/bundler/lib/bundler/man/bundle-gem.1.ronn b/bundler/lib/bundler/man/bundle-gem.1.ronn
index 61c741fb24f6..6169175f3c04 100644
--- a/bundler/lib/bundler/man/bundle-gem.1.ronn
+++ b/bundler/lib/bundler/man/bundle-gem.1.ronn
@@ -41,7 +41,7 @@ configuration file using the following names:
   Do not create a `CODE_OF_CONDUCT.md` (overrides `--coc` specified in the
   global config).
 
-* `--ext`:
+* `--ext=c`:
   Add boilerplate for C extension code to the generated project. This behavior
   is disabled by default.
 
diff --git a/bundler/spec/commands/newgem_spec.rb b/bundler/spec/commands/newgem_spec.rb
index 55a04b69c59f..c4e8b3a6e766 100644
--- a/bundler/spec/commands/newgem_spec.rb
+++ b/bundler/spec/commands/newgem_spec.rb
@@ -312,28 +312,28 @@ def bundle_exec_standardrb
 
   it "has no rubocop offenses when using --ext and --linter=rubocop flag", :readline do
     skip "ruby_core has an 'ast.rb' file that gets in the middle and breaks this spec" if ruby_core?
-    bundle "gem #{gem_name} --ext --linter=rubocop"
+    bundle "gem #{gem_name} --ext=c --linter=rubocop"
     bundle_exec_rubocop
     expect(last_command).to be_success
   end
 
   it "has no rubocop offenses when using --ext, --test=minitest, and --linter=rubocop flag", :readline do
     skip "ruby_core has an 'ast.rb' file that gets in the middle and breaks this spec" if ruby_core?
-    bundle "gem #{gem_name} --ext --test=minitest --linter=rubocop"
+    bundle "gem #{gem_name} --ext=c --test=minitest --linter=rubocop"
     bundle_exec_rubocop
     expect(last_command).to be_success
   end
 
   it "has no rubocop offenses when using --ext, --test=rspec, and --linter=rubocop flag", :readline do
     skip "ruby_core has an 'ast.rb' file that gets in the middle and breaks this spec" if ruby_core?
-    bundle "gem #{gem_name} --ext --test=rspec --linter=rubocop"
+    bundle "gem #{gem_name} --ext=c --test=rspec --linter=rubocop"
     bundle_exec_rubocop
     expect(last_command).to be_success
   end
 
   it "has no rubocop offenses when using --ext, --ext=test-unit, and --linter=rubocop flag", :readline do
     skip "ruby_core has an 'ast.rb' file that gets in the middle and breaks this spec" if ruby_core?
-    bundle "gem #{gem_name} --ext --test=test-unit --linter=rubocop"
+    bundle "gem #{gem_name} --ext=c --test=test-unit --linter=rubocop"
     bundle_exec_rubocop
     expect(last_command).to be_success
   end
@@ -1322,13 +1322,32 @@ def create_temporary_dir(dir)
 
     include_examples "generating a gem"
 
-    context "--ext parameter set" do
-      let(:flags) { "--ext" }
+    context "--ext parameter with no value" do
+      context "is deprecated", :bundler => "< 3" do
+        it "prints deprecation when used after gem name" do
+          bundle ["gem", "--ext", gem_name].compact.join(" ")
+          expect(err).to include "[DEPRECATED] Option `--ext` without explicit value is deprecated."
+          expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.c")).to exist
+        end
+
+        it "prints deprecation when used before gem name" do
+          bundle ["gem", gem_name, "--ext"].compact.join(" ")
+          expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.c")).to exist
+        end
+      end
+    end
+
+    context "--ext parameter set with C" do
+      let(:flags) { "--ext=c" }
 
       before do
         bundle ["gem", gem_name, flags].compact.join(" ")
       end
 
+      it "is not deprecated" do
+        expect(err).not_to include "[DEPRECATED] Option `--ext` without explicit value is deprecated."
+      end
+
       it "builds ext skeleton" do
         expect(bundled_app("#{gem_name}/ext/#{gem_name}/extconf.rb")).to exist
         expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.h")).to exist