The deal will automatically extend for a year, until July 2019, if the 32-year-old utility man triggers an unspecified appearance clause.
"Chris has been a stalwart of this club for a long time," said boss Tony Pulis.
"He is a valued and popular member of the squad. He has shown great character to return from his injury."
Brunt, who can play full-back, winger or in midfield, has scored 45 goals in 331 appearances for Albion since arriving from Sheffield Wednesday in 2007.
He had to undergo knee surgery last March after an injury which forced him out of Euro 2016, but has made a full recovery this season.
West Bromwich Albion's Northern Ireland international Chris Brunt has signed a new extended contract tying him to The Hawthorns until at least 2018.