Tarsal tunnel syndrome (TTS) is a compression neuropathy of the posterior tibial nerve in the ankle, commonly caused by a mass lesion such as ganglion, tumor, fracture, or post-traumatic fibrosis. Although anomalous muscle is relatively common in the ankle, there has been only one report to date implicating it in TTS. This case demonstrates MR findings of a strained flexor digitorum accessorius longus muscle as an unusual cause of TTS.