The Bulgarian forward moves to Craven Cottage after falling down the pecking order at United, with Wayne Rooney, Robin van Persie, Danny Welbeck and Javier Hernandez all ahead of him.
"I'm delighted to have signed for Fulham and I look forward to playing under Martin Jol once again," the striker, 31, told Fulham's website.
As soon as I was aware of Fulham's interest my decision to join was an easy one to make. I'm impressed with the set-up here and the people have made me feel welcome.
"There is a close and personal feel around the club and I can't wait to get started."
Berbatov will wear the number nine shirt for the Cottagers.