The flexor digitorum accessorius longus (FDAL) is the most common accessory muscle found in the posterior compartment of the ankle area. The accessory muscle in this area such as the FDAL may be incidentally identified on magnetic resonance images (MRI). There are some case reports about the FDAL in cadaver researches. However, the MRI findings about the FDAL have been reported in a few cases. In this paper, we report a case of the FDAL incidentally identified on MRI with review of the literature about the FDAL.The flexor digitorum accessorius longus (FDAL) is the most common accessory muscle found in the posterior compartment of the ankle area. The accessory muscle in this area such as the FDAL may be incidentally identified on magnetic resonance images (MRI). There are some case reports about the FDAL in cadaver researches. However, the MRI findings about the FDAL have been reported in a few cases. In this paper, we report a case of the FDAL incidentally identified on MRI with review of the literature about the FDAL.