Johnson was one of the Blues' longest serving players having joined the Midlands club in March 2002 from Blackburn.
He told the Pilgrims official website: "I was made aware of Plymouth's interest over the weekend.
"I've come down here and I want to give it 100%, and do well. I've been frustrated at Birmingham this season and, like any player, I want to play.
"I'm hopefully going to get the opportunity to play down here and I look forward to the opportunity to do well for Plymouth."