Reguilon made his Los Blancos breakthrough last season, coming into the first team under Santiago Solari, who preferred the youngster over Marcelo.
Comunicado Oficial: Reguilón.#RealMadrid— Real Madrid C.F.⚽ (@realmadrid) July 5, 2019
The 22-year-old struggled for game time after Zinedine Zidane's return as head coach, however, making two LaLiga appearances following Solari's departure.
Madrid spent big in June to bring in Ferland Mendy from Lyon for an initial €48million, a move that seemed likely to further limit Reguilon's chances of regular football next season.
And Sevilla have taken advantage, signing the full-back on loan for the 2019-20 campaign.
Reguilon will link up with former Madrid coach Julen Lopetegui in Seville and becomes the club's eighth signing of the transfer window.