He leaves after the 4-0 home defeat by Chelsea on Saturday dropped West Brom to 17th, one point off the relegation zone.
Pulis had been at the club since January 2015.
"These decisions are never taken lightly but always in the interests of the Club," said West Brom chairman John Williams in a statement.
"We are in a results business and over the back end of last season and this season to date, ours have been very disappointing."
Assistant head coach, Gary Megson, has agreed to take charge of first-team affairs until further notice.
West Brom, who are on a 10-match run without a top-flight victory, are next in Premier League action at Tottenham Hotspur on Saturday 25 November.
Pulis is the fifth managerial departure in the Premier League this season, after Frank de Boer (Crystal Palace), Craig Shakespeare (Leicester City), Ronald Koeman (Everton) and Slaven Bilic (West Ham United).