The hosts, who were rooted to the foot of the Premier League table and showing few signs of lifting off the bottom, fell behind inside the opening minute when Djed Spence freed James Maddison to fire home after 37 seconds.
That was only the beginning of their nightmarish first half, though, as they found themselves 3-0 down before the game was even 15 minutes old.
Son Heung-Min converted Maddison’s cross at the far post before Dejan Kulusevski marked his 100th Premier League appearance by slamming home from six yards.
Spurs were simply cutting through Southampton at will, and after squandering a healthy lead against Chelsea last weekend, Ange Postecoglou’s side were in no mood to leave this one to chance and added a fourth inside the opening half-hour when Pape Matar Sarr jinked his way into the box and found the corner.
At 4-0, the game was well beyond Russell Martin’s side, but they did come close to restoring some pride before the break when Adam Armstrong turned Ryan Manning’s cross inches wide of the target.
Never before in Premier League history had Spurs scored five goals in the first half, but that record toppled here when Maddison squeezed an effort into the roof of the net from the tightest of angles.
Swathes of Saints fans had already headed for the exits by the time the half-time whistle went, but those who did missed their best period of the game, as they managed to negotiate the opening stages of the second half without conceding again, and the fact that was a small win in itself shows just how bad they were before the interval.
Those who had remained inside St Mary’s Stadium thought they had something to cheer about when Mateus Fernandes stooped to head home what would have been scant consolation, but the assistant referee raised his flag to compound their misery.
Spurs ended up simply toying with Southampton in the closing stages and could’ve added more if they wanted, but after recent struggles, Postecoglou will be glad to leave the south coast with three points.
Russell Martin and Co., meanwhile, have just five points to their name this term and must find a solution quickly.
Flashscore Man of the Match: James Maddison (Tottenham Hotspur)