Chronic lateral ankle pain and instability can be associated with a split in the peroneus brevis tendon. This case reports a peroneus brevis split that was centered over the sharp posterior edge of the fibula and was associated with laxity of the superior peroneal retinaculum, and chronic ankle instability. The mechanism of the split was easily demonstrated during surgery by everting the foot and pulling longitudinally on the peroneus longus tendon. The interrelationship of lateral ankle instability with superior retinacular laxity and resultant peroneus brevis splits can account for posttraumatic lateral ankle pain. Surgical treatment must identify and correct the underlying pathology and should attempt to repair or debride the peroneus brevis tendon, reconstruct the superior peroneal retinaculum, flatten the posterior edge of the fibula by removing the sharp bony prominence, and address any associated lateral ankle instability with either a modified Chrisman-Snook, Anderson, or modified Brostrom-Gould procedure.