The former Vitesse Arnhem midfielder's move was rubberstamped last night.
"I'm very happy to be here at such a nice club with great players,'" he told chelseafc.com. "I am really happy, the stadium is good and the training ground is fantastic.
"I know about all the players and will meet them on Monday.
"I expect a lot, I've watched matches and it's really nice to be here."