The Frenchman has finally sealed his move to the Camp Nou side after spending the past five seasons at Valencia.
The Catalans were looking to sign a new center back following the retirement of Carles Puyol at the end of the 2013-14 campaign and has long been pursuing 30-year-old Mathieu.
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After protracted negotations, Barca has finally secured the signing of the Frenchman, who will have a €50 million buyout clause in his new contract after becoming one of the most expensive players at the age of 30 or over in history.
"Barcelona and Valencia have reached an agreement for the transfer of the French player Jeremy Mathieu," a statement on the club's official website reads.
"Jeremy Mathieu will sign a contract with Barcelona for the next four seasons plus an optional season more.
"The cost of the transfer is €20 million and the buyout clause will be €50 million."
Mathieu started his professional career with Sochaux and also wore the Toulouse jersey before joining Valencia in the summer of 2009.
The defender had since made 177 appearances in all competitions for the Mestalla side, scoring seven goals in the process.