The former Chelsea and Arsenal forward was a free agent, having left Brazilian club Corinthians earlier this month.
Transfer deadline day: Willian returns to West London as Bamba Dieng lands in England. Willian, 34, has already been training with Fulham ahead of sealing the move.
Also, the West London club are bringing in Dan James in a loan move from Leeds, according to talkSPORT.
The Wales international was also a subject of interest from Tottenham this summer.
Fulham also had a bid for Ben Brereton Diaz rejected by Blackburn on the deadline day.
Meanwhile, Marseille forward Bamba Dieng is on his way to Leeds to undergo a medical after the two clubs agreed a fee.
Leeds are also interested in Wolves striker Hwang Hee-Chan.