The fibularis tertius muscle, also known as the peroneus tertius, is located on the front of the lower leg. At the top it is attached in the lower third of the fibula, one of the two bones of the lower leg. The tendon at the lower end is attached to the metatarsal bone of the fifth toe. Nervous system functions are supplied to this muscle by the deep fibular nerve.

Its function is to move the toes toward the shin (dorsiflexion), and to move the foot away from the middle plane of the body (eversion).

Problems with this muscle can manifest as ankle and heel pain. Massage therapy for the fibularis tertius muscle can provide relief, as can hot or cold compresses. Non-steroidal anti-inflammatory drugs (NSAIDS) such as Ibuprofen can reduce inflammation and provide pain relief. The fibularis tertius may be used in reconstructive surgery to replace muscle tissue elsewhere in the body.

The fibularis tertius muscle is rarely found in other primates. It is thought that its development played a role in the evolution of bipedalism (walking on two limbs).