The 28-year old has undergone a medical and held talks with Wolves officials whilst the two clubs have agreed a fee believed to be £7million.
Johnson told Sky Sports News that he would be departing St Andrew's after joining from Cardiff City two years ago, saying a deal would be completed soon.
"We're halfway there," he said.
"It wasn't an easy decision, I enjoyed my time at Birmingham but relegation half made the decision for me.
"I want to be in the Premier League and I'm sure everyone can appreciate that. But I enjoyed my time at Birmingham."