The 25-year-old arrived at the Stadium of Light on a six-month loan in January and was a key player as Sunderland won the Championship playoffs after finishing fourth in the table.
Le Fee has penned a four-year deal and will cost a reported £19.3 million after a clause in his loan deal was activated once Sunderland sealed promotion.
"I said before the play-off final that my heart was in Sunderland - I wanted to stay and this made it the most important game of my life. Together, we did it," Le Fee told the club website.
"From the moment I arrived, I have enjoyed my football, my team-mates and our ers. We shared an incredible experience and now we get to take the next step."
Le Fee had previously worked with Sunderland boss Regis Le Bris at Lorient before ing Rennes and a big money move to Roma less than 12 months ago.
However, he made just 10 appearances for the Italian giants before being loaned out.
