diff --git a/include/ruby/encoding.h b/include/ruby/encoding.h
index 32d874aa9d..93939ee7db 100644
--- a/include/ruby/encoding.h
+++ b/include/ruby/encoding.h
@@ -124,6 +124,7 @@ int rb_enc_get_index(VALUE obj);
 void rb_enc_set_index(VALUE obj, int encindex);
 int rb_enc_capable(VALUE obj);
 int rb_enc_find_index(const char *name);
+int rb_enc_alias(const char *alias, const char *orig);
 int rb_to_encoding_index(VALUE);
 rb_encoding *rb_to_encoding(VALUE);
 rb_encoding *rb_find_encoding(VALUE);
diff --git a/spec/ruby/optional/capi/encoding_spec.rb b/spec/ruby/optional/capi/encoding_spec.rb
index cd3f00d478..dc1019e8ae 100644
--- a/spec/ruby/optional/capi/encoding_spec.rb
+++ b/spec/ruby/optional/capi/encoding_spec.rb
@@ -37,9 +37,9 @@ describe "C-API Encoding function" do
     @s = CApiEncodingSpecs.new
   end
 
-  describe "rb_encdb_alias" do
+  describe "rb_enc_alias" do
     it "creates an alias for an existing Encoding" do
-      @s.rb_encdb_alias("ZOMGWTFBBQ", "UTF-8").should >= 0
+      @s.rb_enc_alias("ZOMGWTFBBQ", "UTF-8").should >= 0
       Encoding.find("ZOMGWTFBBQ").name.should == "UTF-8"
     end
   end
diff --git a/spec/ruby/optional/capi/ext/encoding_spec.c b/spec/ruby/optional/capi/ext/encoding_spec.c
index 6f4057865b..87b97f3a1b 100644
--- a/spec/ruby/optional/capi/ext/encoding_spec.c
+++ b/spec/ruby/optional/capi/ext/encoding_spec.c
@@ -93,12 +93,11 @@ static VALUE encoding_spec_rb_default_external_encoding(VALUE self) {
 }
 #endif
 
-#ifdef HAVE_RB_ENCDB_ALIAS
-/* Not exposed by MRI C-API encoding.h but used in the pg gem. */
-extern int rb_encdb_alias(const char* alias, const char* orig);
+#ifdef HAVE_RB_ENC_ALIAS
+extern int rb_enc_alias(const char* alias, const char* orig);
 
-static VALUE encoding_spec_rb_encdb_alias(VALUE self, VALUE alias, VALUE orig) {
-  return INT2NUM(rb_encdb_alias(RSTRING_PTR(alias), RSTRING_PTR(orig)));
+static VALUE encoding_spec_rb_enc_alias(VALUE self, VALUE alias, VALUE orig) {
+  return INT2NUM(rb_enc_alias(RSTRING_PTR(alias), RSTRING_PTR(orig)));
 }
 #endif
 
@@ -326,8 +325,8 @@ void Init_encoding_spec(void) {
                    encoding_spec_rb_default_external_encoding, 0);
 #endif
 
-#ifdef HAVE_RB_ENCDB_ALIAS
-  rb_define_method(cls, "rb_encdb_alias", encoding_spec_rb_encdb_alias, 2);
+#ifdef HAVE_RB_ENC_ALIAS
+  rb_define_method(cls, "rb_enc_alias", encoding_spec_rb_enc_alias, 2);
 #endif
 
 #ifdef HAVE_RB_ENC_ASSOCIATE
diff --git a/spec/ruby/optional/capi/ext/rubyspec.h b/spec/ruby/optional/capi/ext/rubyspec.h
index 4f50450dfb..1ece9541c7 100644
--- a/spec/ruby/optional/capi/ext/rubyspec.h
+++ b/spec/ruby/optional/capi/ext/rubyspec.h
@@ -204,7 +204,7 @@
 #define HAVE_RB_DEFAULT_INTERNAL_ENCODING  1
 #define HAVE_RB_DEFAULT_EXTERNAL_ENCODING  1
 
-#define HAVE_RB_ENCDB_ALIAS                1
+#define HAVE_RB_ENC_ALIAS                  1
 #define HAVE_RB_ENC_ASSOCIATE              1
 #define HAVE_RB_ENC_ASSOCIATE_INDEX        1
 #define HAVE_RB_ENC_CODEPOINT_LEN          1