The 21-year-old has previously spent time at Torquay, Bournemouth, MK Dons and Leeds, and Spurs boss Andre Villas-Boas has decided to ship the youngster out to Millwall to gain more first-team experience.
The right-back has England international Kyle Walker and Kyle Naughton in front of him in the White Hart Lane pecking order, and Villas-Boas feels Smith will benefit from more playing time elsewhere.
The young fullback has five caps for Stuart Pearce's England under-21 side, but has so far failed to crack the Spurs first-team.
Smith came through the Tottenham youth academy, and has been at the club since 2007. But the youngster has only made one Premier League appearance for the north London club.
Smith enjoyed a successful spell on loan at League 1 Bournemouth in 2010, and a year later in the same division with MK Dons.
And the 21-year-old showed he has what takes to play in the Championship when he joined Leeds United briefly on a temporary basis earlier this year, before Tottenham recalled him.
The next step for Smith is to prove his worth in the Premier League, and Villas-Boas will hope another successful spell away from Spurs will help the youngster develop into a top player.