diff --git a/go/errorutil/errors.go b/go/errorutil/errors.go
index 5a9988f8..4926a72e 100644
--- a/go/errorutil/errors.go
+++ b/go/errorutil/errors.go
@@ -97,6 +97,11 @@ func PermissionDenied(format string, args ...any) error {
 	return connect.NewError(connect.CodePermissionDenied, fmt.Errorf(format, args...))
 }
 
+// Unimplemented creates a new Unimplemented error (Connect Code 12) with a formatted error message.
+func Unimplemented(format string, args ...any) error {
+	return connect.NewError(connect.CodeUnimplemented, fmt.Errorf(format, args...))
+}
+
 // NewNotFound creates a new Not Found error (Connect Code 5) from an underlying error.
 func NewNotFound(err error) error {
 	return connect.NewError(connect.CodeNotFound, err)
@@ -142,6 +147,11 @@ func NewPermissionDenied(err error) error {
 	return connect.NewError(connect.CodePermissionDenied, err)
 }
 
+// NewUnimplemented creates a new Unimplemented error (Connect Code 12) from an underlying error.
+func NewUnimplemented(err error) error {
+	return connect.NewError(connect.CodeUnimplemented, err)
+}
+
 // IsNotFound reports whether the error is a Not Found error (Connect Code 5).
 func IsNotFound(err error) bool {
 	connectErr := Convert(err)
@@ -196,6 +206,12 @@ func IsPermissionDenied(err error) bool {
 	return connectErr.Code() == connect.CodePermissionDenied
 }
 
+// IsUnimplemented reports whether the error is a Unimplemented error (Connect Code 12).
+func IsUnimplemented(err error) bool {
+	connectErr := Convert(err)
+	return connectErr.Code() == connect.CodeUnimplemented
+}
+
 // WrapConnectErr wraps the error into a connect.Error with the given code if it is not already a connect.Error.
 // If the error is already a *connect.Error, it returns it unchanged.
 func WrapConnectErr(c connect.Code, underlying error) *connect.Error {
diff --git a/go/errorutil/errors_test.go b/go/errorutil/errors_test.go
index 6869cff0..ca3134bf 100644
--- a/go/errorutil/errors_test.go
+++ b/go/errorutil/errors_test.go
@@ -76,6 +76,11 @@ func TestIsErrFns(t *testing.T) {
 			errFn: IsUnauthenticated,
 			code:  connect.CodeUnauthenticated,
 		},
+		{
+			name:  "unimplemented",
+			errFn: IsUnimplemented,
+			code:  connect.CodeUnimplemented,
+		},
 	} {
 		tests := []struct {
 			name    string