Beckham's son Brooklyn, 12, collects replica kits and asked his dad to have a word with striker Mario, despite David being a legend with rivals United.
The Italian star, 21, duly obliged, handing over the shirt to Becks as his side lost 3-2 at home to United in the FA Cup on Sunday.
A source told The Sun: "Brooklyn has an impressive collection of kits from some of the world's top players. He's a big fan of Balotelli and really wanted a signed shirt.
"Despite City being United's rivals, Becks didn't mind as he's also a big admirer of Balotelli."
The ex-England captain, 36, watched the match with Brooklyn, younger son Romeo, nine, and dad Ted.