The Brazilian was subject of interest from Liverpool and underwent a medical at Tottenham Hotspur before eventually sealing his £30m move to Stamford Bridge.
However, the playmaker revealed that the Blues were always his preferred destination.
"I've always been following Chelsea and every time I came to London I would try to see a Chelsea game and see how the team was doing because it's been my dream to come to play here," Willian told the club's official website.
"Finally it's become a reality, and when the offer came I never gave a thought to anybody else, I said to myself that I would run here, and I'm happy to be here now. Chelsea was always my first choice.
"It's going to be an immense pleasure to work with Jose Mourinho, listen to him and learn from him. I'm sure I'm going to grow as a professional and I'm looking forward to training."
Willian will be unavailable for Friday's Super Cup clash against Bayern Munich.