Willian became the latest addition to Chelsea's attacking options after completing his move from Anzhi Makhachkala on Wednesday.
Although he had been on the verge of joining Premier League rival Tottenham, Willian signed a five-year deal at Chelsea after being granted a British work permit. Financial terms of the deal were not disclosed but British media reported that it was worth £32 million pounds ($50 million).
"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, so I'm very happy and looking forward to it," the 25-year-old Brazil international said, referring to Chelsea manager Jose Mourinho.
Willian, who can play on the wing or behind the striker, joined Anzhi from Ukrainian side Shakhtar Donetsk in January but the big-spending Russian club's owner recently decided to slash the budget.
The Brazilian attacking midfielder is Chelsea's third big signing after Germany forward Andre Schuerrle and Netherlands midfielder Marco van Ginkel.
Willian, who learned his skills at Corinthians and has made two appearances for Brazil, joined Shakhtar Donetsk in August 2007, winning four league titles, three Ukrainian Cups, three Ukrainian Super Cups and the 2009 UEFA Cup.
Willian had been close to joining Tottenham following his medical at the North London club. But Chelsea offered him a better deal after owner Roman Abramovich reportedly intervened to secure the deal.