"These decisions are never taken lightly but always in the interests of the club," said chairman John Williams on West Brom's official website. "We are in a results business and over the back end of last season and this season to date, ours have been very disappointing.”
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He added: "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. We wish him well in his future endeavours."
Assistant head coach Gary Megson has agreed to take charge of first-team affairs until further notice.
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Owner Guochuan Lai watched the Chelsea defeat after flying over from China for a pre-arranged trip and is now tasked with finding a new manager, with Aberdeen boss Derek McInnes linked to the post.
Pulis, 59 — the fifth Premier League manager to be sacked this season — replaced Alan Irvine when the Baggies were a point above the relegation zone in January 2015, eventually leading them to 13th.
They finished 14th in 2015-16 and came 10th last season but failed to win any of their final nine matches.
Pulis made a defence of his record before the Chelsea defeat, suggesting Albion would struggle to get a better replacement.
"If I left tomorrow I'd put my record on the table and put it in front of anybody and see what they think of what I've done here in three years," he said on Friday.
Pulis, who boasts a proud record of never being relegated as a manager, has previously been in charge at Crystal Palace and had two stints at Stoke, where he reached the FA Cup final and qualified for Europe during his second spell at the club.
The number of top-flight sackings was already at a five-season high even before the Welshman's exit, with four up to November 20 compared with the next highest mark of three by the same stage in the 2015-16 season.
The other bosses fired so far this season are Frank de Boer (Crystal Palace), Craig Shakespeare (Leicester), Ronald Koeman (Everton) and Slaven Bilic (West Ham).
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