Western mathematical tradition has long attributed the discovery of this theorem to Pythagoras. More recent historical research has found that the theorem was known to the Babylonians, circa 1500 BC, so long before Pythagoras. The Chinese knew him perhaps around 1100 BC and the Hindus probably around 500 BC.
Boyer, C.B., History of Mathematics. São Paulo, Edgard Blücher Publisher, 1996.
Eves, H., Introduction to the History of Mathematics. Campinas, UNICAMP Publisher, 1995.