Reguilón, who was currently without a club after his contract with Premier League team Tottenham expired this previous summer, joins Inter Miami as a free agent.
He will occupy an international roster slot, with the hopes of replacing recently retired Jordi Alba on the pitch.
"It's a very ambitious project, a winning club that is doing things the right way, and that's what attracted me: coming here to keep winning and competing," Reguilón said in a statement. "My objective is to keep winning, go after the trophies we're missing, and win everything here."
Reguilón joins Major League Soccer after experience in LaLiga, recording 56 appearances in the Spanish league after completing spells with Real Madrid, Atlético Madrid and Sevilla.
The defender formed part of Sevilla's 2019-20 squad that lifted the UEFA Europa League trophy. The left back also boasts 81 appearances in the Premier League through time with Tottenham, Manchester United and Brentford FC.
Reguilón stands as Inter Miami's first new signing of the transfer window, following their MLS Cup triumph on Dec. 6.
The roster, however, continues to evolve ahead of the 2026 campaign after the club announced end-of-the-year decisions on Dec. 11.
Inter Miami confirmed ongoing negotiations with Luis Suárez, after the player's one-year contract extension expired following the end of the season.
Team co-owner Jorge Mas revealed Suárez will be the one to decide his future with Inter Miami, as the door remains open to him for an additional contract.
"Luis deserves to be able to make that decision to be able to leave through the front door and be celebrated like he should be by the club," Mas told reporters.
"And if he decides to stay at the club for another year, it would be great. I would like to see Suárez stay."
The club also announced ongoing negotiations with Allen Obando, Rocco Ríos Novo, Baltasar Rodríguez and Tadeo Allende, before saying goodbye to Fafà Picault, William Yarbrough, Ryan Sailor and Marcelo Weigandt.
