The 26-year old Scotland midfielder has been a key player for Albion since joining from Middlesbrough in 2007 and will now remain at The Hawthorns until the summer of 2016.
"This is my sixth season here now. I'm committed to the club, I feel settled here. There was no reason for wanting to leave," he told the Birmingham Mail.
"I'm 26, the longest-serving player and I quite like that. Hopefully other players like Chris Brunt and Jonas will follow."
He added: "It feels like home. That's down to playing every week, the team playing well, being settled.
"The fans have been great with me, the new manager coming in has been a big factor in me signing."