The defender has risen through the ranks at Real Madrid in recent seasons, having originally joined the club as a youth player in 2005 at age eight.
Aside from two loans to lower-division side UD Logrones from 2015-17, Reguilon has primarily featured for Real Madrid Castilla until this past season.
Reguilon made 22 total appearances for Real Madrid in 2018-19, including his professional debut on October 2, 2018 in a Champions League loss to CSKA Moscow before his league debut one month later in a Liga win over Real Valladolid.
The defender came into the first team under Santiago Solari, who preferred the youngster over Marcelo.
The 22-year-old struggled for game time after Zinedine Zidane's return as head coach, however, making two La Liga appearances following Solari's departure.
Madrid spent big in June to bring in Ferland Mendy from Lyon for an initial €48 million, 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.
Sevilla's sporting director Monchi, however, said on Friday that Sevilla are yet to receive any offers for the 28-year-old, who hit 18 LaLiga goals last season.
"There's nothing new. He has one week left of holiday and will report for pre-season on the 13th [of July] to travel to the United States," he told reporters.
"From then on, the [transfer] market is open and there are possibilities for any player of our squad. As of today, no offer has arrived."
Sevilla finished sixth in La Liga last season, and will play in the Europa League in their upcoming campaign.