Ridgewell is currently having talks over personal terms at the Baggies' training ground with a view to completing the move before tonight's transfer deadline.
Roy Hodgson said: "I expect the Ridgewell deal to be completed later this afternoon once he has completed a medical.
"He was a transfer target last summer and since the start of January.
"Negotiations have been long and arduous, which is often the case, but I am pleased to say the deal should go through - subject to the medical."
Albion had an initial bid rejected for the former Aston Villa player and head coach Hodgson indicated that he was prepared to "move on" from any potential deal.
However, their latest offer has now been accepted by the Championship play-off contenders.
Ridgewell made it clear last summer he wanted to leave St Andrew's after handing in a transfer request. He will provide competition for Nicky Shorey at left-back and can also operate as a central defender.
Ridgewell moved from Villa to Blues nearly five years ago to secure regular first-team football and was a member of last season's Carling Cup-winning team.
But Ridgewell made it clear he wanted to remain in the Premier League following Birmingham's relegation last May.
He will link up at The Hawthorns with goalkeeper Ben Foster, who is on loan from Blues.
