Spurs dominated throughout but failed to find a way through a resolute home defence and past the in-form Baggies goalkeeper Ben Foster.
Tony Pulis’ side looked to have secured a smash-and-grab win when former Spurs man Nacer Chadli struck with eight minutes remaining.
However, Alli fired into the net in the final minute of normal time to ensure they secured a share of the spoils which leaves them a point behind league leaders Manchester City and Arsenal.
West Brom are 10th with 10 points from eight games.
Spurs assumed control from the first whistle but it took until the 11th minute for their first half-chance when Erik Lamela picked up a return pass from Vincent Janssen inside the area but the Argentine flashed his effort across the face of goal.
Despite continuing to keep possession, Mauricio Pochettino’s side struggled to create any more clear openings until the 31st minute but Lamela scuffed his volley in the area which allowed Foster to gather.
Alli then turned and fired wide from eight yards, before Foster pulled off a fine one-handed save to push away Ben Davies’ effort on 35 minutes.
Spurs continued to press and Alli was set up by Janssen down the left side of the area but the midfielder’s effort was well saved by Foster.
Foster came to the rescue moments before the interval when Christian Eriksen’s deflected 25-yard effort looked set to drop into the bottom corner until the big keeper got a vital hand to push the ball wide of the post.
West Brom had the final effort of the half with James McClean’s effort parried by Hugo Lloris, who managed to get the ball clear before Salomon Rondon could tap-in the rebound from close range.
It was West Brom who had the first chance of the second half when a corner fell to Gareth McAuley in the area but he couldn’t force the ball into the net from close range.
Spurs were hit by the loss of Toby Alderweireld, who had to be carried off on a stretcher, but they remained in control.
On 62 minutes Moussa Sissoko hit an effort which looked set to go well wide until it took a wicked deflection and dropped just past the Albion goal.
Spurs were made to pay for not taking advantage of all their chances and possession in the 82nd minute when McClean forced a fine save from Lloris only for the loose ball to drop to Chadli who followed up to fire into the roof of the net.
However, the visitors rescued the point their play deserved in the 89th minute when Eriksen saw his effort blocked and he then opted to set up Alli who threaded his shot into the far corner despite having several defenders in front of him.
Spurs almost snatched it the final minute of injury time but again Foster rescued Albion as he clawed away Eriksen’s 25-yard free kick.