The giant American company's brand name will feature on the club shirt for four years. No terms of the sponsorship deal have been announced but the deal is thought to be worth about £80 million.
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 Barclays Premier League champions described the bond with Aon as "a significant global partnership and principal sponsorship agreement".
Aon are the world's leading risk advisor and human capital consultant and Gill added: "The announcement clearly strengthens our position as one of the biggest clubs in world football.
"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."