The Belgium forward has completed a move to link-up with former Blues coach Steve Clarke at the Hawthorns after finding opportunities limited during his first season in England
Chelsea striker Romelu Lukaku has joined West Brom on a season-long loan.
The Belgium international joined the Blues for a reported £18 million in 2011 but has struggled to establish himself in the first team, making just one Premier League start last season.
Lukaku will wear the No.20 shirt for the Baggies but will not feature in Friday's friendly against Nottingham Forest.
"I'm delighted to welcome Romelu to the club," Steve Clarke told the club's official website.
"He's a strong, powerful centre-forward who is young, enthusiastic and eager to learn. He was also desperate to come here.
"Romelu had a lot of Europe's top clubs chasing him when he joined Chelsea from Anderlecht for big money a year ago and also had several Premier League teams wanting to take him on loan this summer, so we've done well to get him.
"We're more than happy for Romelu to continue his development with us. I am sure he will be a big help to the squad over the course of the season."
Clarke sympathised with the highly rated youngster’s attempts to break into the first team squad at Stamford Bridge, and believes a consistent run of games will see the striker start to show the promise that led to Chelsea paying out such a large fee 12 months ago.
Clarke added: "With the calibre of strikers Chelsea have got, he's found it difficult to break into the first team at Stamford Bridge.
"But he's had a season working at a top Premier League club and he now needs more minutes on the pitch.”
The Antwerp-born forward rose to prominence with Anderlecht in the Jupiler League in Belgium, making his debut as a 16-year-old.