Sesostris III (sĭsŏsˈtrĭs) [key], d. 1840 B.C., king of ancient Egypt, of the XII dynasty. He succeeded (1878 B.C.) his father Sesostris II. He fixed the southern boundary of Egypt above the Second Cataract of the Nile not far from Wadi Halfa and curbed the power of the Egyptian nobles. His son Amenemhet III was coregent and then his successor.