An orthodontic retainer is configured for application to a lingual surface of a patient's teeth. The retainer is custom designed to exactly match the shape of the lingual side of the patient's teeth and is formed from a plurality of tooth-adhering portions connected by elongated connection portions. The tooth-adhering portions each have a generally flat top edge and a generally curved bottom edge. The connection portions are located at a position below the top edge and above the bottom edge of the tooth-adhering portions. The retainer is formed as a single piece by milling, injection molding or 3D printing.
