Unai Simón grew up in the Basque city of Gasteiz and was enrolled at the academy of Aurrera Vitoria at a young age. He joined Lezama in 2011 at the age of 14 and immediately began to turn heads. As he rose through the ranks he started to stand out as one of the best young goalkeepers in the country and has since featured for the Spain U19 and U21 teams. Simón was promoted to Bilbao Athletic in 2016 where he became the starter and even had call ups to the first team due to injuries in the squad. He got the opportunity to train with the senior squad in the summer of 2017 and 2018.
After being sent on loan with Elche for the 2018/2019 season, he was recalled in mid-August due to an injury to Iago Herrerín. He made his first team debut at the San Mamés against Leganés in the first game of the season.
Unai Simón is under contract through 2023 and his deal contains a €50 million release clause.