Flexor digitorum tendon entrapment is a known complication of proximal phalangeal fractures and is commonly diagnosed clinically. However, it can sometimes be subtle and may not be suspected initially. A case of proximal phalangeal fracture of the index finger complicated by entrapment of the flexor digitorum profundus tendon is presented in which the diagnosis was made based on sonographic findings subsequently confirmed by CT and later surgical findings. This case demonstrates that tendon entrapment complicating phalangeal fractures can be diagnosed using sonography.