Simón has spent his entire club career with Athletic, developing with their youth academy, farm team and reserves and making his senior debut in 2018, amassing 200 appearances by 2025.
Simón made his senior international debut for Spain in 2020 and played atUEFA Euro 2020, the2022 FIFA World Cup, andUEFA Euro 2024, where he featured in all but one match as Spain won the latter tournament.
On 2 June 2017, he was called up to the main squad for the pre-season by new first team managerJosé Ángel Ziganda,[9] but continued to appear exclusively for the B-team. On 13 July 2018, he signed a contract extension until 2023,[10] and was loaned toSegunda División sideElche CF two weeks later.[11]
On 15 August 2018, afterKepa Arrizabalaga's departure from the club andIago Herrerín's injury, Simón was recalled by Athletic before he had played a match for Elche.[12] He initially appeared to be behind the olderÁlex Remiro in the queue for selection,[13] but made his professional – andLa Liga – debut five days later, starting in a 2–1 home win againstCD Leganés.[14][15] In just his third appearance in the top division, Simón played in a 1–1 draw withReal Madrid where he made several key saves and was named player of the match.[16] However, when Herrerín regained full fitness in October 2018 he was made the starting goalkeeper, with Simón's only appearances over the subsequent four months coming in theCopa del Rey.[7]
For the2019–20 season, Simón was given the No. 1 squad number and began the campaign as first choice, keeping a clean sheet in an opening day win overFC Barcelona.[17] He continued his strong form throughout the season, missing two fixtures with illness and completing 33 matches in the league, conceding 29 goals[18] to finish third in theZamora Trophy rankings.[19] On the 37th, penultimate, matchday, he was sent off during the first half against Leganés for a 'last man' challenge outside the penalty area, with the resultant suspension ending his season a game early; with ten men, Athletic Bilbao lost 2–0 to end their slim hopes of qualifying for theUEFA Europa League.[20]
In August 2020, he signed a new contract with the club running to the summer of 2025, with no buyout clause.[21][22] In January 2021, he saved a penalty fromGetafe'sJaime Mata.[23] The following month he had a decisive role in the quarter-finals of the Copa del Rey, stopping two of the three penalties in the shootout at theEstadio Benito Villamarín againstReal Betis.[24][25]
During the 2021–22 season, he kept his place in the starting lineup and had several impressive performances, the most notable being againstLevante and Getafe (both matches ending goalless).[26][27] In the following season, with the arrival ofErnesto Valverde, he maintained his role as an undisputed starter in the league and performed at a good level.[28][29][30] On 10 November 2023, in "one of his best moments as a professional",[31] he saved a penalty fromCelta de Vigo strikerIago Aspas to keep the score level in a game his team went on to win.[32] On 16 March 2024, he stopped another penalty fromLuis Rioja ofAlavés, starting a move which culminated in a goal for Athletic forty seconds later as they again went on to win;[33] in addition, he consolidated his position as leader in the Zamora Trophy with his 15th clean sheet in the league.[34][35] Having been the starter for Copa del Rey final defeats in2020 and2021, he played no active part as the clubfinally won the trophy in April 2024, watching from the bench as understudyJulen Agirrezabala was selected for all rounds of the tournament. At the end of the season, Simón extended his contract to 2029.[36]
In October 2024, Simón was ranked in second place for theYashin Trophy.[37] He was unavailable for the first four months of the2024–25 season due to rehabilitation after surgery on his wrist, which was carried out immediately after the Euros tournament ended.[38] He made his 200th appearance for Athletic Bilbao in January 2025.[39]
On 1 September 2017, Simón debuted with the under-21 team againstItaly.[41] He was selected for the2019 UEFA European Under-21 Championship, playing the first game of the group stage as Spain once more finished as champions (Sivera was again first-choice).[42][43]
Simón received his first call-up toSpain senior team on 20 August 2020, for the2020–21 UEFA Nations League matches againstGermany andUkraine.[44] Despite having made an error which led to a goal at club level a few days earlier, he debuted in a friendly match againstthe Netherlands on 11 November, playing the entire match.[45]
On 24 May 2021, Simón was included inLuis Enrique's24-man squad forUEFA Euro 2020.[46] Having displacedDavid de Gea as first-choice, he started all of the team's matches, and caused anown goal in the 5–3 round of 16 victory againstCroatia when he was unable to control a long backwards pass byPedri.[47] Luis Enrique praised his mentality for having recovered from that mistake.[48] He eventually saved two penalties as Spain won the shootout againstSwitzerland in the quarter-finals after a 1–1 draw,[5] for which he was also awarded the Star of the Match.[49] The semi-final againstItaly ended with the same score, and Simón saved the first attempt in the shootout byManuel Locatelli, but the Italians prevailed.[50] Simón was also called up for theSpanish Olympic team for thedelayed 2020 tournament in Japan.[51] With Simón as the starting goalkeeper they reached the final, but lost toBrazil.[52]
Simón was the starter at the2022 FIFA World Cup, in which Spain progressed from their group with a victory, draw, and defeat – before losing 3–0 on penalties toMorocco in theround of 16 after a goalless draw. He saved fromBadr Benoun, but the three of teammates failed with their efforts.[53] Spain had better fortune in the2023 UEFA Nations League final one year later, where another goalless draw againstCroatia was followed by a 5–4 victory in the shootout to claim the title, Simón blocking attempts fromLovro Majer andBruno Petković.[4][54]
On 7 June 2024, Simón was named in Spain's squad forEuropean Championships the same year. He started and played all matches except the final group stage match. In the latter, Simón played the full 90 minutes ofthe final as Spain beat England to become European champions for the fourth time.[55]