The 23-year-old, who enjoyed spells on loan at Valencia and Stuttgart during his time on the books at Stamford Bridge, has signed a three-year contract with Ronald Koeman’s side.
Romeu becomes Southampton's seventh signing of a busy summer transfer window, joining Cuco Martina, Jordy Clasie, Cedric Soares, Juanmi and loanees Maarten Stekelenburg and Steven Caulker at St Mary's.
“It’s very good to be a part of this club,” Romeu told Southampton's official website. “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.”
