Wolves are staging a press conference at their Compton training ground to announce the signing of the former Cardiff player after he agreed personal terms and passed a medical.
All that remains is for Johnson to formally put pen to paper on a four-year contract ahead of facing the media.
Wolves boss Mick McCarthy has been tracking Johnson since the end of season when Birmingham's relegation from the Premier League was confirmed.
Johnson moved from Cardiff to St. Andrew's during the summer of 2009 in a £5million deal and formed an effective partnership with Scott Dann during his two years at Birmingham .
McCarthy will hope Johnson can help to shore up a defence which was guilty of leaking too many soft goals last season.
The deal will take the Wolves boss' spending to £12million this summer, with midfielder Jamie O'Hara having moved from Tottenham in a £5million deal.
Goalkeeper Dorus de Vries moved to Molineux from Swansea on a Bosman transfer.