diff --git a/activerecord/lib/active_record/nested_attributes.rb b/activerecord/lib/active_record/nested_attributes.rb
index 39a0e31e79cdc..a127545a6af0c 100644
--- a/activerecord/lib/active_record/nested_attributes.rb
+++ b/activerecord/lib/active_record/nested_attributes.rb
@@ -260,21 +260,11 @@ def _destroy
       marked_for_destruction?
     end
 
-    # Deal with deprecated _delete.
-    #
-    def _delete #:nodoc:
-      ActiveSupport::Deprecation.warn "_delete is deprecated in nested attributes. Use _destroy instead."
-      _destroy
-    end
-
     private
 
     # Attribute hash keys that should not be assigned as normal attributes.
     # These hash keys are nested attributes implementation details.
-    #
-    # TODO Remove _delete from UNASSIGNABLE_KEYS when deprecation warning are
-    # removed.
-    UNASSIGNABLE_KEYS = %w( id _destroy _delete )
+    UNASSIGNABLE_KEYS = %w( id _destroy )
 
     # Assigns the given attributes to the association.
     #
@@ -371,8 +361,7 @@ def assign_to_or_mark_for_destruction(record, attributes, allow_destroy)
 
     # Determines if a hash contains a truthy _destroy key.
     def has_destroy_flag?(hash)
-      ConnectionAdapters::Column.value_to_boolean(hash['_destroy']) ||
-      ConnectionAdapters::Column.value_to_boolean(hash['_delete']) # TODO Remove after deprecation.
+      ConnectionAdapters::Column.value_to_boolean(hash['_destroy'])
     end
 
     # Determines if a new record should be build by checking for
diff --git a/activerecord/test/cases/nested_attributes_test.rb b/activerecord/test/cases/nested_attributes_test.rb
index de6ac62f03244..f9ece340774df 100644
--- a/activerecord/test/cases/nested_attributes_test.rb
+++ b/activerecord/test/cases/nested_attributes_test.rb
@@ -81,12 +81,6 @@ def test_a_model_should_respond_to_underscore_destroy_and_return_if_it_is_marked
     assert ship._destroy
   end
 
-  def test_underscore_delete_is_deprecated
-    ActiveSupport::Deprecation.expects(:warn)
-    ship = Ship.create!(:name => 'Nights Dirty Lightning')
-    ship._delete
-  end
-
   def test_reject_if_method_without_arguments
     Pirate.accepts_nested_attributes_for :ship, :reject_if => :new_record?