There were chances at both ends at the Riverside Stadium in a largely entertaining game that had surprisingly lacked goals given both teams are in good form.
There looked like being no breakthrough as Middlesbrough, now unbeaten in their opening five games, claimed the points to move above Leeds courtesy of Daniel Ayala's stoppage-time winner.
West Brom, who hit seven past QPR in their last outing, could not find a way past Boro's defence, which has now kept four clean sheets in a row despite the sale of Ben Gibson to Burnley for £15million.
Home boss Pulis named latest recruit Mo Besic on the bench. His arrival on loan from Everton on Thursday was completed in time for him to be named alongside fit-again Jordan Hugill, who signed earlier this month from West Ham, among the substitutes for the first time.
West Brom, a team with whom Pulis enjoyed success before leaving in November 2017, knew what sort of style Middlesbrough would play with and the presence of Ayala at the back for the first time this season increased the height of a towering defensive unit.
The Baggies had the closest of the earlier efforts, though, even if it was from one of the defenders playing alongside Ayala at the back.
Aden Flint got in a muddle as he attempted to block a dangerous delivery from Harvey Barnes and the former Bristol City man's outstretched boot turned the cross against his own bar. Goalkeeper Darren Randolph was beaten.
Other than that it was Middlesbrough who had the best of the opportunities in the opening period.
Stewart Downing's drive from 25 yards forced a one-handed save from Sam Johnstone, while Jonny Howson and Martin Braithwaite saw efforts go wide.
Braithwaite had the best of the lot before half-time. He somehow missed the target with a half volley from inside six yards when Flint's knock down from Howson's deep cross fell perfectly for the Danish forward.
After the restart there was more of the same. Ryan Shotton's back-post header flew over after he was picked out by an excellent run and cross from Britt Assombalonga, while Braithwaite's header was well saved by Johnstone.
West Brom had their moments, too. The best was when Dwight Gayle, on loan from Newcastle, fired over the bar when a two-on-one situation ended with him being rolled in by Matt Phillips.
But then Ayala struck late on when he brought down a deep free-kick to the back post before poking a finish inside the far corner to seal the points for Middlesbrough.
