Medial foot pain is a common complaint in rehabilitation clinics. The differential diagnoses include many musculoskeletal disorders like tendonitis and inflammation of ossicles. Posterior tibialis tendonitis is a common cause of foot pain in adults. The accessory navicular (AN) bone is occasionally observed and considered as a secondary ossification center of the navicular bone. Occasionally, posterior tibialis tendonitis and AN bone may cause acute or chronic medial foot pain with varying degrees of dysfunction. Previously, the diagnosis of an AN bone in a painful medial foot was based on clinical presentation and radiographic examinations such as plain radiography, bone scintigraphy, and magnetic resonance imaging. However, the application of soft-tissue ultrasonography for the diagnosis of posterior tibialis tendonitis associated with an AN bone has not been documented. Here, we report the case of a 60-yr-old woman with painful medial foot which had a diagnosis of posterior tibialis tendonitis associated with an AN bone by high-resolution ultrasonography.