A malleolus is the bony prominence on each side of the human ankle.

Each leg is supported by two bones, the tibia on the inner side (medial) of the leg and the fibula on the outer side (lateral) of the leg. The medial malleolus is the prominence on the inner side of the ankle, formed by the lower end of the tibia. The lateral malleolus is the prominence on the outer side of the ankle, formed by the lower end of the fibula.

The word malleolus (/məˈliːələs, mæ-/), plural malleoli (/məˈliːəˌlaɪ, mæ-/), comes from Latin and means "small hammer". (It is cognate with mallet.)

The medial malleolus is found at the foot end of the tibia. The medial surface of the lower extremity of tibia is prolonged downward to form a strong pyramidal process, flattened from without inward - the medial malleolus.

The major structure that passes anterior to the medial malleolus is the saphenous vein.

Structures that pass behind medial malleolus deep to the flexor retinaculum:

The lateral malleolus is found at the foot end of the fibula, of a pyramidal form, and somewhat flattened from side to side; it descends to a lower level than the medial malleolus.

A major structure that is located between the lateral malleolus and the Achilles tendon is the sural nerve.