The move comes just two weeks after the 34-year-old midfielder raked teammates over the coals in a scathing interview with United's specialty pay-per-view channel.
The interview was so damning that it was deemed 'unbroadcastable' by Manchester United TV.
"At all times I have endeavoured to do my best for the management and the team," Keane said. "Whilst it is a sad day for me to leave such a great club and manager, I believe that the time has now come for me to move on."
Roy Keane is leaving Manchester United after 12 1/2 seasons. (CP File Photo)
Considered the inspirational leader and on field general of a United team that dominated English soccer in the 1990s, Keane is now free to join another club.
The Irishman had indicated at the beginning of the current season that he was likely to leave United at the end of this year when his contract expired.
Keane joined United in 1993 on a transfer from Nottingham Forest and went on to become one of the greatest players in the team's illustrious history.
He became captain in 1997 when Frenchman Eric Cantona retired. Regarded as of the best midfielders in the world, Keane was renowned for tough tackling, expert passing and physical play. He helped United win seven Premiership titles and four FA Cups. He scored 51 goals in 480 games for United.
"The best midfield player in the world of his generation, he is already one of the great figures in our clubs illustrious history," United manager Sir Alex Ferguson said of Keane. "Roy has been central to the success of the club in the last 12 1/2 years and everyone at Old Trafford wishes him well in the rest of his career and beyond."
