The Netherlands global spent last season on loan at Crystal Palace, where he made 21 appearances in all competitions.
Fulham do not have an option to buy.
Fulham had a busy transfer Deadline Day, as Fosu-Mensah finalised his switch to the club alongside the likes of Joe Bryan, Luciano Vietto, and Sergio Rico.
The Netherlands global, who has made 21 senior appearances for United since his debut under Louis van Gaal in 2016, played 24 games for Palace last season.
Fulham appears well suited for the rigors of the Premier League and should be able to benefit from the support of the home crowd in their first game back since 2014.
"We arrived last in the Premier League (through the play-offs), so our principal target has to be to stay up", said Jokanovic, who in his previous job in England guided Watford to promotion to the Premier League but then left.
Goalkeeper Fabri is touted to make his first start, while Callum Chambers will make his debut at the heart of the defence.
"I'm very happy to come to the club and sign". "I'm excited to start to train and above all enjoy a season that starts on Saturday".