The gambling company has replaced Betfred with the three-year deal. As part of the contact, Betfair will use the Manchester United logo in marketing activity.
One of its first marketing initiatives is the 'Betfair Goal Challenge'. United strikers including Wayne Rooney and Michael Owen will compete to score the most goals over the coming season. Each League strike will earn £1,000 for the Manchester United Foundation.
The betting firm has also launched website www.betfairfootball.com , which will include content from Manchester United players.
The news follows its rival 118Bet's announcement that it is the official betting partner of Chelsea FC.