Manchester United signed him on loan in the summer but then activated a break clause in his contract at the beginning of the month as Luke Shaw and Tyrell Malacia near returns from injury.
It is understood that 27-year-old Reguilon had initially preferred a loan deal back in his native Spain this month and was considering his options, but Brentford made left-back their number one priority this window and have moved fast to beat other potential suitors to Reguilon’s signature.
The six-time Spanish international joined Spurs from Real Madrid in 2020, and made 12 appearance for United before his loan stint was cut short.
Reguilon has not been in Tottenham’s plans under manager Ange Postecoglou, and he is likely to get a large amount of game time at Brentford, where the only nominal left-back is Aaron Hickey.
Hickey, 21, is currently out with a thigh injury and will not play until February.