The Baggies keeper arrived at the Hawthorns initially on loan from Birmingham in 2011 and has gone on to make 161 appearances for the club.
Foster, 33, has been an ever-present in the top-flight this campaign and his fine form has helped Tony Pulis’ men up to eighth place in the table.
Baggies boss Pulis said: “We’re delighted we’ve extended Ben’s contract.
“His form this season has been there for all to see and he continues to set the standards at the club all the goalkeepers have to aspire to.”
Forster added: “It was a really easy decision. It was not an option for me to think about going somewhere else.
“When the club came and said they wanted to offer me a new deal it was a pretty simple process. It is my club now. My kids are West Brom fans, my family are West Brom fans. That is the way it is.
“It is a good place to be around at the minute.”