The 30-year-old, who joined the club from Hull City in 2010, has impressed for the Baggies this season while deputising for Ben Foster.
Sporting and technical director Richard Garlick told the club's official website: "We've seen this season it is important to have a good number two. Boaz has definitely contributed to our success and stepped up to the plate when Ben Foster was injured.
"In the Barclays Premier League it is vital you have a number two you can trust in but also a number two that can push your number one. Everyone at the club has faith in both of them and, with Luke Daniels, we've got a good set of three keepers who push each other in training and have a good understanding with each other.
"It is testament to the work Dean Kiely has done that we have such a strong unit where all the keepers are happy to support each other no matter who is playing."
Myhill has made 20 appearances for the club.