The left-back has been deemed surplus to requirements by manager Antonio Conte, and has been training away from the first-team squad during the summer.
Reguilon, who joined from Real Madrid two years ago, is heading back to the Spanish capital after a deal, without a buy option, was reached between the two clubs.
Atletico left-back Renan Lodi is set to join Nottingham Forest on loan, leaving a space for Reguilon to take up.
Reguilon rose to prominence on loan at Sevilla, before Spurs bought him from his parent club Real Madrid.
It is the fourth Spurs first-team member to head out on loan in August, after Joe Rodon, Giovani Lo Celso and Tanguy Ndombele.
Harry Winks understood to be closing in on a similar move to Sampdoria after initially holding out for possible Premier League interest.