Flexor digitorum accessorius (quadratus plantae) is a muscle of second layer of the sole of the foot. Though it is not a very important muscle in terms of movements or stability of foot, it could pose problems when it presents variations. We observed a novel, accessory slip of flexor digitorum accessorius. The slip arose from the fascia covering abductor digiti minimi muscle and got inserted into the tendon of the flexor digitorum longus muscle. It was supplied by a branch from lateral plantar nerve. The proximal part of this accessory slip surrounded the lateral plantar nerve and vessels. Compression of lateral plantar nerve by this accessory slip might produce symptoms similar to that of tarsal tunnel syndrome and lead to misdiagnosis. The case could be of importance to foot and ankle surgeons and radiologists.