Pulis has secured 13th and 14th place finishes in his two Premier League campaigns with the Baggies, and they are 13th this term after 10 points from nine matches.
"Tony has huge experience and proven know-how in one of the most demanding leagues in world football," West Brom chairman John Williams told wba.co.uk. "He brings assurance and stability at an important time."
The Welshman previously took Stoke City up to the top flight in 2008 and kept them in the division during five successful PL years at the club, before winning the 2013/14 Premier League Manager of the Season after an escape from relegation with Crystal Palace.
He has managed 281 matches during nine seasons in the Premier League, also winning the Manager of the Month in April 2014 and February 2015.