Brazil attacking midfielder Willian formally joined Chelsea from Russian side Anzhi Makhachkala on Wednesday after the two clubs reached 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 £30 million ($47 million, 35 million euros) 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 Schuerrle.
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 Schuerrle 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.
Speaking before the announcement that an agreement for the transfer had been reached, Mourinho said he felt no sympathy for Spurs despite swiping the player from beneath their noses.
"That's the danger of medicals before contracts. But you have to do the medical before," he said.
"The best thing to do is the medical in secret. If the player is fine, you can sign him. If he's not, you don't have to destroy his career. Sometimes there are sources who find out everything."
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