Sesostris III sĭsŏs´trĭs , d. 1840 BC, king of ancient Egypt, of the XII dynasty. He succeeded (1878 BC) 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.