Barco joins after the earlier arrivals of Morgan Rogers, Danny Welbeck, Emmanuel Emegha, Maxence Lacroix and Marco Palestra, with the fee undisclosed by either club. He will link up with the rest of Chelsea's squad during the remainder of their pre-season tour, ahead of a friendly against Juventus in Hong Kong on August 5.
The 21-year-old made one appearance for Argentina at this summer's World Cup, coming on for a 19-minute cameo in the group-stage win over Jordan as Lionel Scaloni's side went on to finish runners-up to Spain. He also drew attention for an incident after Argentina's semi-final win over England, when Jude Bellingham was seen slapping him following Barco's celebrations in front of the England players.
A product of the Boca Juniors academy, Barco joined Brighton in 2024 for a fee of around £7.9m but made only a handful of appearances on the south coast before spending time on loan at Sevilla and then Strasbourg, where a strong showing convinced the French club to sign him permanently the following summer.
His move to Chelsea is the second this window involving a player going directly from Strasbourg to Stamford Bridge, with both clubs owned by Todd Boehly and Clearlake Capital's BlueCo consortium — making it the 13th transfer between the two clubs since the start of last season.
Primarily a left-back capable of also playing in central midfield, Barco made 53 appearances for Strasbourg in all competitions and helped the club qualify for the Conference League last season.
His versatility gives manager Xabi Alonso another option across multiple positions as he continues to reshape a Chelsea squad already boosted by significant investment this summer, with the club having leaned heavily on its sister-club relationship with Strasbourg to source promising young talent at competitive value.