The 25-year-old had been linked with a move to Premier League rivals Liverpool and Tottenham during the transfer window, but the Stamford Bridge outfit have now won the race for his signature.
The Brazilian was officially presented by the club on Wednesday, after he was granted a work permit to play in England.
Willian told the club's official website: 'I'm very happy to be here, it's been a dream of mine to come and play here.
"Chelsea are one of the best clubs in the world, and now I'm going to play for one of the best managers in the world (Jose Mourinho), so I'm very happy and looking forward to it."
'I love my job. I love being able to play this sport. In the end, there’s something addictive in winning for me' Alexia Putellas speaks about what drove her back from an ACL injury to lift the World Cup
Alexia Putellas: "We’re already the only club to have won the men’s and women’s Champions League, so the only thing even more historic to do than that would be to win it in the same season"
