United confirmed they had agreed a fee with Arsenal on Wednesday evening, and the Dutch striker underwent a successful medical on Thursday before the final elements of the transfer were wrapped up. The deal has been done in time for Van Persie to go straight into Sir Alex Ferguson's squad for Monday's opening Premier League clash at Everton.
United announced in a statement: "Robin van Persie has completed his medical, agreed personal terms and signed for Manchester United on a four-year contract until June 2016."
Manager Ferguson said: "Robin is a world-class striker with a proven record in England and in European football. His talents need no introduction to our fans - he has scored a number of goals against us in some classic battles with his former club.
"His movement, finishing and all-round ability are outstanding."
Van Persie said it was an "honour" to sign for United.
He said: "I am looking forward to following in the footsteps of so many great strikers, bringing my experience and playing my part to help the team compete for the biggest trophies in the game. I can't wait to get started."
The news brings to an end one of the longest-running transfer sagas of the summer. Ferguson has fought off competition from Manchester City and Juventus to land Van Persie.
However, he also needed to break an impasse with Arsenal boss Arsene Wenger, who did not want to sell last season's Golden Boot winner to a rival club.
Even a week ago, Wenger had not given up hope of persuading the 29-year-old to stay, but when Van Persie was booed by Arsenal fans at a pre-season game in Germany on Sunday, it signalled the end of his Gunners career.