Brazil attacking midfielder Willian formally joined Chelsea from Russian side Anzhi Makhachkala on Wednesday after the two clubs reached an agreement last week. The 25-year-old had been set to join Tottenham Hotspur, having undergone a medical at the north London club earlier this week, only for capital rivals Chelsea to hijack the deal.
British media reports claim Chelsea will pay a fee of around £30m to secure Willian's services.
"I'm very happy to be here, it's been a dream of mine to come and play here," Willian told Chelsea's official website on Wednesday. "Chelsea are one of the best clubs in the world and now I'm going to play for one of the best managers (Jose Mourinho) in the world. I'm very happy and looking forward to it," he said.
Willian is the fourth player to join Chelsea during the current transfer window, following Australian goalkeeper Mark Schwarzer, Dutch midfielder Marco van Ginkel and Germany international Andre Schurrle.
His arrival will further swell the number of attacking midfielders at manager Mourinho's disposal, with Eden Hazard, Juan Mata, Oscar, Victor Moses, Kevin De Bruyne and Schurrle already competing for just three starting places.
Willian only joined Anzhi from Ukrainian side Shakhtar Donetsk in January, but he is being moved on after the Russian side's billionaire owner Suleiman Kerimov decided to restructure the club and drastically shrink their budget.