Tarsal navicular stress fractures are injuries more commonly observed in athletes involved in sprinting and jumping sports. Known risk factors for the development of navicular stress fractures include pes cavus, metatarsus adductus, limited subtalar or ankle motion, medial narrowing of the talonavicular joint, as well as a short first metatarsal. The presence of an os supranaviculare has yet to be described as a predisposing factor in the occurrence of navicular stress fractures. We present two cases of navicular stress fractures in patients with an os supranaviculare and discuss possible reasons for such an association.