Having initially impressed on loan at the German side from Everton in the second half of the 2017/2018 season, Lookman joined Leipzig permanently for £22m on a five-year-deal at the start of last season but hardly had a look-in at the Champions League semi-finalists.
The 22-year-old spent most of his time on the Leipzig bench last season, featuring in just 13 games across all competitions.
After sealing his move to Craven Cottage, Lookman told Fulham’s official website: "I'm glad to have it over the line and done now. I'm glad I can be here to help the team.
"After the conversations I had with Scott Parker, I got a great feeling. That's the working relationship I want to have. He's young and hungry and I'm young and hungry as well, so it's a good match."
Lookman has represented England at youth level, winning the FIFA U-20 World Cup three years ago but is still eligible to play for Nigeria, where his parents hail from.
Fulham's sporting director Tony Khan added: "Ademola, in his young career, has gained experience in the Premier League and Bundesliga and contributed in England's 2017 U20 World Cup win, and we believe that he's a very good addition to our squad."
Fulham have started the season poorly, with three straight defeats on their return to the top flight.
