His assistant Gary Megson, the club's former manager between 2000 and 2004, will take over on a caretaker basis.
Saturday's 4-0 hammering at home by champions Chelsea was the final straw for West Brom's Chinese owner Guochuan Lai, who was growing impatient with their disappointing results.
Albion won their first three games of the season in all competitions but their victory over fourth-tier Accrington Stanley 3-1 in the League Cup in August was their last.
Even the pragmatic Pulis, whose teams are usually hard to beat, said he would not be surprised if he was dismissed.
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West Brom are 17th in the standings with 10 points from 12 games, having lost six of their 12 matches.
'VERY DISAPPOINTING'
"We are in a results business and over the back end of last season and this season to date, ours have been very disappointing," West Brom chairman John Williams said.
"These decisions are never taken lightly but always in the interests of the club.
"We would like to place on record our appreciation of Tony's contribution and hard work during a period of transition for the club which included a change of ownership."
West Brom ended last season in 10th position but finished the campaign with one win in their last eight league games.
Pulis said in a statement on Monday that "over the last three years we have achieved a great deal and I am proud to have managed West Bromwich Albion"
"We enjoyed a fantastic season last year, finishing tenth in the Premier League for only the third occasion in Albion's history...
"I strongly believe that this group of players can return to this level of performance and finish in the upper reaches of the Premier League," he added.
Pulis, who has also managed Stoke City and Crystal Palace in the Premier League, became West Brom manager in January 2015 when they were 17th and helped them finish in 13th place.
Pulis is the fifth Premier League manager to lose his job this season after Palace's Frank de Boer, Leicester City's Craig Shakespeare, Everton's Ronald Koeman and West Ham United's Slaven Bilic were all sacked.
(Reporting by Aditi Prakash in Bengaluru and Ian Chadband; Editing by Gareth Jones and Ken Ferris)
