He'll be staying with the club until the summer of 2021, with the club holding the option for an additional year.
In the clubs statement, he said: “I’m delighted to be at the club for another few years and now I can’t wait to kick on.
“You need the manager’s faith and a chance to play football – and he gave me that pretty early on. It’s been nice to get a few more goals this season and that’s a part of my game I want to keep improving.
“The progress this club has made in the past few years shows that we’re only moving in one direction. Hopefully we can make another step this season and our aim at the moment is to get into the Championship.”
Blues boss Kenny Jackett added: “We’re delighted that Ben has signed a new contract and committed his future to the club.
“It was frustrating for him when he was out of the side because of the form of Ben Thompson earlier in the season, but since then he’s become a key midfielder for us.
“It’s great when a homegrown player establishes himself in the team and I have every confidence that Ben will continue to improve with us going forward.”