The former Arsenal and Chelsea winger, who turned 34 last month, has joined on a free transfer after leaving Brazilian club Corinthians and has signed a one-year contract.
Willian’s arrival was part of a busy deadline day for Fulham, who also signed Leeds winger Dan James and Paris Saint-Germain defender Layvin Kurzawa on a season-long loan deals.
Striker Carlos Vinicius, who previously spent time on loan at Tottenham, also joined late last night as part of a deal worth £4.3million from Benfica.
Newly-promoted Fulham, who have made a good start to the season and are eighth in the table with eight points from five games, are next in action away at Spurs tomorrow, when Willian hopes to make his debut.
“I hope that I can be involved in this game now — I am really excited to start playing,” said the Brazilian. “The Premier League for me is the best league in the world.
“I think it is a good opportunity for me to come back to the Premier League and to show that I can still play here, still play in a high level.
“I think it doesn’t matter the age, you can always improve yourself. For me, I think that way.
“I think I can improve every day. I still want to fight for good things, I have a desire, I want to compete, I still want to play in a high level, so that’s why I am here.”