The Frenchman is set to sign a four-year deal at the Nou Camp and will provide competition for Jordi Alba on the left side of defense.
"FC Barcelona and Valencia CF have reached agreement for the transfer of the French player, Jeremy Mathieu," read a statement on www.fcbarcelona.com.
"Jeremy Mathieu will sign a contract with FC Barcelona for the next four seasons, plus an optional season more. The cost of the transfer is €20 million (£15.8million) and the buyout clause will be 50million euros."
Mathieu traveled with the Valencia squad to Madrid as a stop off on their way to Peru as part of their preseason tour, but will now travel to Barcelona to sign his contract with the Catalans and join Luis Enrique's side for pre-preseason training.
The Frenchman came through the ranks at Ligue 1 side Sochaux, making his professional debut in 2002 and eventually moved to fellow French side Toulouse in 2005.
Four years later he swapped France for Spain to join Valencia, where he alternated between the left-back spot and centre-half, excelling in the latter position last season once Juan Antonio Pizzi took charge.
Although Mathieu will be able to deputize for Alba, he is likely to also play as defender and cover for the loss of captain Carles Puyol, who retired from football at the end of last season.
Mathieu is the first central defender Barca have signed since Dmitro Chigrinski, who joined the club from Shakhtar Donetsk in 2009 for 25million euros only to return to the Ukranian side a year later following an unsuccessful stay at the Nou Camp.
The signing of 30-year-old Mathieu, who has two international caps with France, has attracted criticism from former Barcelona defender Rafael Marquez, who told Catalan radio station RAC1: "I don't think he has the profile to play for Barcelona and even less so for this price."
However, Barca midfielder Rafinha Alcantara welcomed the signing of Mathieu, telling a press conference on Wednesday: "Mathieu is a complete defender, with incredible speed. He's a great defender."