John Maurice of Nassau (Dutch: Johan Maurits van Nassau-Siegen, German: Johann Moritz von Nassau-Siegen, Portuguese: João Maurício de Nassau-Siegen, 17 June 1604 – 20 December 1679), called "the Brazilian", was count and (from 1674) prince of Nassau-Siegen, and Grand Master of the Order of Saint John (Bailiwick of Brandenburg).

He was born in Dillenburg. His father was John VII of Nassau; his grandfather John VI of Nassau, the younger brother of Dutch stadtholder William the Silent of Orange, making John Maurice a grandnephew to William the Silent.

John Maurice joined the Dutch army in 1621, at a very early age. He distinguished himself in the campaigns of his cousin, the stadtholder Frederick Henry, Prince of Orange. In 1626 he became captain. He was involved in 1629 in the capture of Den Bosch. In 1636, he conquered a fortress at Schenkenschans.

He was appointed as the governor of the Dutch possessions in Brazil in 1636 by the Dutch West India Company on recommendation of Frederick Henry. He landed at Recife, the port of Pernambuco and the chief stronghold of the Dutch, in January 1637.

By a series of successful expeditions, he gradually extended the Dutch possessions from Sergipe on the south to São Luís de Maranhão in the north. With the assistance of the famous architect, Pieter Post of Haarlem, he transformed Recife by building a new town adorned with public buildings, bridges, channels and gardens in the then Dutch style, later naming the newly reformed town Mauritsstad, after himself.

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