Internacional says the 33-year-old striker signed a contract until July 2015 and will be introduced to fans on Saturday.
He had been playing with Inter Milan since transferring to the Italian club from Atletico de Madrid in 2011. He also previously played for Manchester United and Villareal.
Forlan is one of the top foreign players to be signed by a Brazilian club. Internacional won the Copa Libertadores, Latin America's most important club competition, in 2006 and 2010.
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