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 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.