Reguilon will provide cover for the injured Luke Shaw, who is expected to be out for 10 weeks.
Reguilon could make his debut against Arsenal on Sunday after being registered in time.
Reguilon said he couldn’t turn down the chance to join United and is ready to help the team achieve success.
Manager Erik ten Hag went into the market for a left back after injuries to Shaw and Tyrell Malacia.
United initially targeted Chelsea’s Marc Cucurella but a deal stalled after he played for Chelsea in the Carabao Cup.
Reguilon was allowed to have a medical at United on Thursday and completed the paperwork.
Ten Hag said Reguilon brings valuable experience from playing for big clubs in La Liga and the Premier League.
The loan signing provides cover after the latest injury blow to Luke Shaw.