The Spanish defensive midfielder has joined the Saints on a three-year deal for an undisclosed fee believed to be around £5million.
Romeu, 23, arrives at St Mary's having joined the Blues from Barcelona in 2011. During his time at Stamford Bridge, the former Spain U21 and U23 international had loan spells with Valencia and Stuttgart.
"It's very good to be a part of this club," Romeu told the club's official website after putting pen to paper.
"I was looking for a good club and for something new in my career. After spending two years on loan, I needed something permanent and a club with a future - and I think I chose the right one.
"Southampton are doing the right things in the last few years and, once they came to me and asked me to come here, from the first day I was hoping to come.
"Now I just want to keep going and to keep developing as a player, and I'm looking forward to start playing as soon as possible."
Very delighted to announce my new signing for @Southamptonfc. Can't wait to start playing & wearing the #Saints shirt pic.twitter.com/8zrbcb8IyA
— Oriol Romeu (@OriolRomeu) August 12, 2015
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!