The ex-England youth international spent 11 years at the Etihad and was let go earlier this summer.
“I'm delighted," he's told the club's website. “I was desperate for a new challenge and I think this is the perfect place for me.
“I had a taste of Scottish football from my time at St Mirren and I really enjoyed it. It's very competitive but I think that suits me."
Plummer has penned a 12 month deal.