Diagnosis of subtle Lisfranc injuries can be difficult for the examining clinician. Radiographs do not always reveal the full extent of tarsometatarsal injuries. We describe a clinical sign found in significant Lisfranc injuries that consists of a mid-foot ecchymotic area on the plantar aspect. This implies the potential for significant injury to the plantar tarsometatarsal ligaments. We call this the plantar ecchymosis sign. Patients with this injury and sign should be aggressively evaluated, with early consideration for stress radiographs and ORIF if indicated, as the prognosis of the undertreated Lisfranc injury is poor.