Having spent 10 years working his way through the age groups at Real Madrid’s Academy, Reguilón made his senior debut for Segunda Division B side UD Logrones during a loan spell in 2015/16. He was recalled to Real Madrid in January 2016 and played for Real Madrid Castilla during the second half of the season. Reguilón then returned to UD Logrones for the 2016/17 campaign, making 31 appearances and scoring eight goals in Spain’s third tier.

Having earned a promotion to Real Madrid’s first-team squad, Reguilón made his senior bow in October 2018, in the Champions League, and made his La Liga debut one month later.

Reguilón was loaned to Sevilla for the 2019/20 season. He played 38 matches across all competitions, registering three goals and five assists. Sevilla won the Europa League that term, with Reguilón named in the competition's Squad of the Season. Reguilón moved to Tottenham Hotspur on a permanent deal in September 2020. He made 36 appearances in all competitions during his first season with the north London club, assisting six goals and notably crossing for Moussa Sissoko to score Spurs' opening goal in their Carabao Cup semi-final win over Brentford in January 2021.He played 31 times during the following campaign, missing the latter part of the season due to a groin injury. The defender joined Atlético Madrid on loan for 2022/23, but injury restricted him to 12 appearances and he returned to Spurs in the summer of 2023. Reguilón made a temporary switch to Manchester United in September 2023. He played 12 games for United during the first half of this season and showcased an appetite to contribute in offensive areas, averaging 0.9 key passes per game.

On the international stage, Reguilón made his debut for Spain on 6 September 2020, registering an assist in a 4-0 UEFA Nations League victory over Ukraine. He has earned a further five caps for his country since then.