The Spanish champions have not disclosed how much they have paid for the 31-year-old.
Romeu came through Barcelona's famed La Masia academy and made two appearances for the senior team in the 2010/2011 season before joining Chelsea in the summer of 2011.
Romeu struggled to become a regular at Stamford Bridge and after spending loan spells at Valencia and Stuttgart, he joined Southampton on a permanent deal in 2015.
Romeu played 256 times for Southampton in his seven years on the South Coast before returning to Spain last season to help Girona finish 10th on their return to the top flight.
A statement from Xavi Hernandez's side read: "Barcelona and Girona FC have reached an agreement for the transfer of the player Oriol Romeu.
"The player will sign a contract with the Club for the next three seasons, until 30 June 2026, and his buy out clause is set at 400 million euros."
Romeu is Barcelona's third summer signing after Ilkay Gundogan and Inigo Martinez joined on free transfers from Manchester City and Athletic Bilbao respectively.
