diff --git a/CHANGELOG.md b/CHANGELOG.md
index aff41c93ad12..86e7fb10d3e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,7 @@ Compatibility:
 * Support passing anonymous * and ** parameters as method call arguments (#3039, @andrykonchin).
 * Handle either positional or keywords arguments by default in `Struct.new` (#3039, @rwstauner).
 * Add `rb_enc_interned_str_cstr` function (#3408, @goyox86, @thomasmarshall).
+* Add `rb_str_to_interned_str` function (#3408, @thomasmarshall).
 
 Performance:
 
diff --git a/lib/cext/ABI_check.txt b/lib/cext/ABI_check.txt
index b4de39476753..48082f72f087 100644
--- a/lib/cext/ABI_check.txt
+++ b/lib/cext/ABI_check.txt
@@ -1 +1 @@
-11
+12
diff --git a/spec/ruby/optional/capi/ext/string_spec.c b/spec/ruby/optional/capi/ext/string_spec.c
index 390c5a8ab2be..9c13d805c5ca 100644
--- a/spec/ruby/optional/capi/ext/string_spec.c
+++ b/spec/ruby/optional/capi/ext/string_spec.c
@@ -589,6 +589,10 @@ static VALUE string_spec_rb_enc_interned_str_cstr(VALUE self, VALUE str, VALUE e
   return rb_enc_interned_str_cstr(RSTRING_PTR(str), e);
 }
 
+static VALUE string_spec_rb_str_to_interned_str(VALUE self, VALUE str) {
+  return rb_str_to_interned_str(str);
+}
+
 void Init_string_spec(void) {
   VALUE cls = rb_define_class("CApiStringSpecs", rb_cObject);
   rb_define_method(cls, "rb_cstr2inum", string_spec_rb_cstr2inum, 2);
@@ -691,6 +695,7 @@ void Init_string_spec(void) {
   rb_define_method(cls, "rb_str_locktmp", string_spec_rb_str_locktmp, 1);
   rb_define_method(cls, "rb_str_unlocktmp", string_spec_rb_str_unlocktmp, 1);
   rb_define_method(cls, "rb_enc_interned_str_cstr", string_spec_rb_enc_interned_str_cstr, 2);
+  rb_define_method(cls, "rb_str_to_interned_str", string_spec_rb_str_to_interned_str, 1);
 }
 
 #ifdef __cplusplus
diff --git a/spec/ruby/optional/capi/string_spec.rb b/spec/ruby/optional/capi/string_spec.rb
index eef170e78bf9..48a06bfdcd38 100644
--- a/spec/ruby/optional/capi/string_spec.rb
+++ b/spec/ruby/optional/capi/string_spec.rb
@@ -1252,4 +1252,20 @@ def inspect
       result1.should_not.equal?(result2)
     end
   end
+
+  describe "rb_str_to_interned_str" do
+    it "returns a frozen string" do
+      str = "hello"
+      result = @s.rb_str_to_interned_str(str)
+      result.should.is_a?(String)
+      result.should.frozen?
+    end
+
+    it "returns the same frozen string" do
+      str = "hello"
+      result1 = @s.rb_str_to_interned_str(str)
+      result2 = @s.rb_str_to_interned_str(str)
+      result1.should.equal?(result2)
+    end
+  end
 end
diff --git a/src/main/c/cext/string.c b/src/main/c/cext/string.c
index 4c7e0be48b46..211af65bb87a 100644
--- a/src/main/c/cext/string.c
+++ b/src/main/c/cext/string.c
@@ -442,3 +442,7 @@ VALUE rb_enc_interned_str_cstr(const char *ptr, rb_encoding *enc) {
   VALUE str = rb_enc_str_new_cstr(ptr, enc);
   return rb_fstring(str);
 }
+
+VALUE rb_str_to_interned_str(VALUE str) {
+  return rb_fstring(str);
+}