The new deal also includes an automatic option for the Basque club to extend his contract further, underlining their confidence in a player whose progress through the club's development system has been rapid.
Monreal, 21, joined Athletic's youth structure in the summer of 2024 from CF Ardoi, initially arriving at Basconia for the 2024-25 campaign. His performances soon attracted attention, with his physical presence, defensive authority and composure making him one of the club's emerging prospects.
His opportunity with the senior side came during a difficult period for Athletic Club's defence last season, when injuries left Ernesto Valverde short of options.
Valverde handed the Pamplona-born defender his first-team debut on February 4, 2026, selecting him in the starting line-up for the Copa del Rey quarter-final against Valencia. Monreal made an immediate impression, earning another start in the first leg of the semi-final against Real Sociedad.
Those appearances represented an important step in his development, demonstrating that he could cope with the demands of senior football despite his limited experience.
Monreal has continued to work towards establishing himself in the first-team picture under new head coach Edin Terzic, who took charge ahead of the current campaign.
The defender completed pre-season with the senior squad, featuring in six friendly matches as he became familiar with Terzic's methods and tactical expectations.
He has also been included in the German coach's matchday squads for both LaLiga and Copa del Rey fixtures this season, although he is yet to make an official appearance.
That lack of competitive minutes has not prevented Athletic from making a clear commitment to his future. The contract extension provides Monreal with security while giving him further time to establish himself among the club's senior defenders.
Athletic have continued to invest in young players who have progressed through their development structure, and Monreal's new agreement is another indication of their intention to retain emerging talent.
For the defender, the next challenge will be turning his growing involvement with the first team into regular opportunities on the pitch. His two Copa del Rey appearances under Valverde have already provided a glimpse of his potential, while the confidence shown by the club through this new contract gives him a platform to continue progressing.
Athletic have now ensured that Monreal's development will remain part of their plans for at least the next three seasons, with the additional extension option offering the club further control over his future.