The Blues have announced that the Brazilian has completed his move to Stamford Bridge following a work permit hearing, while the 25-year-old has expressed delight at the move.
The Brazilian, who had been expected to join Tottenham, has signed a five-year contract with Jose Mourinho's side.
A product of the Corinthians youth academy, the 25-year-old moved to Ukrainian side Shakhtar Donetsk in August 2007 before moving to Russia in January following some eye-catching performances in the Champions League.
"I'm very happy to be here, it's been a dream of mine to come and play here," the player told the Blues' official website upon completing his transfer to the Stamford Bridge outfit.
"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."
Willian has not been named in the squad for Friday's UEFA Super Cup encounter with Bayern Munich in Prague, and in turn will not have the opportunity to make his debut until after the international break, on September 14 against Everton.
The Brazil international is believed to have cost Roman Abramovich's side in the region of 30 million pounds.