The 34-year-old has put pen to paper on a one-year deal with the newly promoted side after terminating his contract with Corinthians.
- Stream on ESPN+: LaLiga, Bundesliga, MLS, more (U.S.)
- Transfer grades: How has your club fared?
"I'm happy to be here, I'm happy to be back in the Premier League," he said in a statement.
"Fulham is a special club, a club that wants to improve, that wants to fight for something bigger, so I'm here to help, and I cant wait to get started."
Willian returns to the English top flight after terminating his deal at Arsenal in August 2021 to return to his native Brazil.
However, sources told ESPN Brasil that he made up his mind to leave Corinthians a month ago and has now sealed a return to English football.
Willian's first stint in England started in 2013 when he joined Chelsea, and he went on to win two Premier League titles, an FA Cup and a League Cup at Stamford Bridge.
At the start of the 2020-21 campaign, he joined London rivals Arsenal on a free transfer but endured a difficult spell, scoring once in 25 appearances.
Fulham will be hopeful his top-flight experience can keep them in the league after winning two and drawing two of their opening five matches.