The company's brand name will feature on the club shirt for four years, replacing current sponsor AIG.
No terms of the sponsorship deal have been announced but it is thought to be worth in the region of £80million.
United chief executive David Gill said: "We are delighted to be entering such an important relationship with a company of the stature of Aon and to have its logo adorn our shirts from the start of the 2010/11 season."
The Premier League champions described the bond with Aon, the world's leading risk advisor and human capital consultant, as 'a significant global partnership and principal sponsorship agreement'.
Shared values"Today's announcement clearly strengthens our position as one of the biggest clubs in world football," Gill continued.
"We look forward to being closely aligned with the world leader in risk management, a firm which shares our values and is an exciting partner for Manchester United."
Greg Case, Aon's president and chief executive officer, said: "It is a unique opportunity when two leaders in their respective fields can come together in a partnership such as the one we are announcing.
"Manchester United has one of the most recognised sports brands in the world. David and his team are all about winning and about excellence; the same holds true for the Aon team.
"While we are delighted that our brand will be showcased to the over 330 million fans of Manchester United as well as the countless followers of football worldwide, we also are extremely excited about the opportunity to maximise the value of this partnership globally."