Romeu came through Barca's youth academy and made his senior team debut in 2010 before moving to Chelsea a year later. The Spainard also had loan spells at Valencia and Stuttgart before spending seven years at Southampton.
He spent a year at Girona where he was a crucial figure clocking up more than 3,000 minutes to guide them to 10th before making his return to Barca, with 20-year-old midfielder Pablo Torre moving the other way on loan.
"Romeu has great experience and knows the club from his previous spell as a blaugrana and the style of play Xavi Hernandez wants to impress on his team," Barca said in a statement.
"The midfielder has shown what he can do in LaLiga and will look to perform right from the off."
The club have inserted a buyout clause of 400 million euros ($449.12 million) in his contract.
Romeu is Barca's fourth signing in the close season after Ilkay Gundogan and Inigo Martinez while Vitor Roque will join the club in January.