Willian was a free agent after terminating his contract with Brazilian side Corinthians last month following death threats from the club's supporters.
Fulham is Willian's third London club after previous spells at Chelsea and Arsenal.
The 34-year-old's bright spell at Chelsea saw him win two Premier League titles, one League Cup, one FA Cup and the Europa League.
However his stint at the Emirates Stadium was a disappointing one as he cut ties with the Gunners just one season into a three-year deal.
‘I’m happy to be here, I’m happy to be back in the Premier League,’ Willian told FFCtv.
‘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 can’t wait to get started.
‘Marco Silva is a very good manager and good guy also. It is a great opportunity for me to come back to the Premier League and show that I can still play at a high level.’
Fulham vice-chairman Tony Khan added: "I’m very happy that Willian has officially signed with Fulham.
"He’s been impressive during his recent training sessions with our squad, and we believe that adding a player of Willian’s quality and someone with his vast experience will strengthen the club as we work to build on the strong start that we’ve had to this Premier League season! Come on Fulham!"
Fulham have started this season in bright fashion, amassing eight points from their opening five games.
