Berchiche and the Basque club have agreed to extend his contract by a further season, taking the deal through to 30 June 2027.
The defender started 35 of his 39 appearances across all competitions last season, underlining his continued importance to Athletic Club's first-team plans despite his advancing years.
The extension means Berchiche will embark on his ninth season at Athletic since arriving from Paris Saint-Germain in 2018-19.
He has made 278 appearances for the club, scoring 15 goals, and has won two major trophies during his time at San Mames — the 2021 Spanish Super Cup and the 2024 Copa del Rey.
Athletic director general of football Mikel Gonzalez praised the decision to extend Berchiche's stay.
"It's great news for the club," Gonzalez said. "With his performance and experience, he is an indispensable player in this squad, as he has shown over these seasons.
"He has demonstrated his professionalism and commitment to the club in a difficult year, and he will surely continue to contribute great things in the new project."
Berchiche's consistency at full-back has provided a rare point of stability for Athletic during a season that fell short of the club's own expectations, with his ability to remain a regular starter well into his thirties continuing to defy the usual pattern for players in his position.
The new deal ensures Athletic retain a player whose experience and leadership qualities are valued highly within the dressing room, even as the club looks to build a squad capable of competing on multiple fronts next season.