The club have been pushing to sign a left-back in recent days, with Luke Shaw and Tyrell Malacia both currently out with injuries to leave Erik ten Hag short of options.
Having been strongly linked with Marc Cucurella, it is Reguilon who makes the move to Old Trafford on a season-long loan deal.
“In life you have to be ready for everything and the chance to represent this great club with such an illustrious history is one that I could not turn down,” he told the club’s official website.
“Having spoken to the manager, I know what he needs from me, and I am ready to play my part in helping the team to achieve success.
“I know that I can contribute to Manchester United this season; I am ready to fight for this group and show everyone my qualities.”
United have no option of obligation to make the move a permanent deal next summer, and will cover the Spaniard’s wages in full for the duration of his loan. There is a break clause in January, which could be activated if United’s injury issues clear by then.
Erik ten Hag confirmed in his press conference on Friday that Reguilon had already trained with his team-mates and would be available to make this debut this weekend, when United travel to face Arsenal on Sunday.
“He is a very experienced player, a player for big clubs, a player that’s played already a lot of games in La Liga, in the Premier League,” Ten Hag said earlier.
“So, he has a very good background, we have seen he can play very intense football, so we are happy. While we had a problem with Luke Shaw injured, Ty[rell] Malacia injured, they are for [the] long term out, so I think we responded very well on that emergency situation.”