In human anatomy, the peroneus longus (also known as fibularis longus) is a superficial muscle in the lateral compartment of the leg, and acts to evert and plantar flex the ankle.

The muscle, the longest and most superficial of the three fibularis muscles, is attached proximally to the head of the fibula and its ‘belly’ runs down most of this bone. It becomes a tendon that goes posteriorly around the lateral malleolus of the ankle, then continues under the foot to attach to the medial cuneiform and first metatarsal.