Johnson put pen to paper on a four-year contract with the midlands club before being unveiled to the media at the club's Compton training ground.
He becomes Wolves's third signing of the summer after midfielder Jamie O'Hara from Tottenham and keeper Dorus de Vries from Swansea.
The formalities of the move were completed on Tuesday when the clubs agreed and Johnson has agreed personal terms and passed a medical.
Wolves manager Mick McCarthy has been a long-time admirer of Johnson and has been tracking him since the end of last season.
Johnson has also made it clear he wants to remain in the top flight since Blues were relegated at the end of last season.
He moved from Cardiff to St. Andrew's during the summer of 2009 in a £5 million deal.
Johnson formed an effective partnership with Scott Dann during his two years at Birmingham.
McCarthy will hope Johnson can shore up a defence which was guilty of leaking too many soft goals last season.
Source: PA
Source: PA