The Scotland international fell out of favour under former coach Steve Clarke, but Pepe Mel's appointment saw him return to the first-team fold.
From March onwards, Dorrans started 10 consecutive games under the Spaniard, scoring twice to help West Brom preserve their top-flight status.
Alan Irvine replaced Mel in readiness for the new campaign and appears to have his compatriot in his plans.
"I'm happy to get my new contract finalised," Dorrans told the club's official website. "We've been in talks for a couple of months and I'm glad to get it over the line.
"It's been a good six years and that's the reason why I've signed the extension. I'm looking forward to getting going now for the new season.
"It was a difficult start for me last season but it was about getting in the team and staying in there, and that's what I’m hoping to do this year.
"I'm aiming to get in at the start of this season and stay in the team for the rest of the campaign."
In his six years at the club the midfielder has made 167 appearances under seven different managers, with Irivine set to be the eighth.
