The 53-year-old heartthrob, who is set to marry British lawyer Amal Alamuddin, apparently asked his 'Monuments Men' co-star to take on the role of best man for his upcoming nuptials, reported Contactmusic.
Earlier reports suggested that George Clooney was planning to ask Brad Pitt.
"A lot of people thought Brad was a certainty to be George's best man as they have been such good friends for years. But he is probably even closer to Matt.
"Besides many years ago, during one drunken night together, George made a promise to Matt that he would make him his best man if he ever got married - so Matt called to claim his prize," a source said.
Although a date for the wedding is yet to be set, Clooney, who proposed to the 36 year-old lawyer last month, is believed to be planning a dream wedding at either his home in Lake Como, Italy or Cabo San Lucas, Mexico, after which the pair will relocate to London so that she can continue to work as a barrister at the Doughty Street Chambers.