Belson, 21, is the fourth new signing made by the Dons this summer, with a permanent deal for Belson in place if he impresses.
Boss Roberto Di Matteo told the Milton Keynes Citizen: "If we had the money it would be a permanent deal, but we don't so it will be a season-long loan.
"He's a midfield player who has come through the youth academy at Metz and looks like a good player."
Meanwhile, the Dons' skipper Keith Andrews has again been linked with a move away from the League One newcomers having sat out the weekend victory over Northampton Town.