The 20-year-old featured in the pre-season for the Red Devils, but has now moved to the Premier League newcomers for the 2018-19 season.
Fosu-Mensah made his Man Utd debut during the 2015-16 campaign.
The Dutchman was on loan at Crystal Palace last season, but there had been suggestions that he would act as cover and competition at Old Trafford - particularly with injury doubts over right-back Antonio Valencia.
Speaking after joining Fulham, Fosu-Mensah told the club's website: "I have a good feeling about the Club".More news: North Korea chides U.S. for pressing sanctions
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Mourinho spoke about how he only sends players out on loan for the sake of their own development and so they're guaranteed minutes.
Fulham have continued their summer overhaul as Timothy Fosu-Mensah joins on a season-long loan from Manchester United. "I'm happy to be here and I want to keep progressing".
Fulham boss Slavisa Jokanovic has been involved in a busy transfer period for the newly-promoted club.
Jean Michael Seri, Aleksandar Mitrovic and Alfie Mawson have all arrived for significant fees, while Andre Schurrle and Calum Chambers have also joined the Cottagers on loan.