The Baggies are closing in on the signature of the former Manchester United player, who is set to initially move on loan with a view to the transfer becoming permanent next summer.
Albion head coach Roy Hodgson has been searching for a new number one keeper since Scott Carson signed for Turkish side Bursaspor last month.
He has been linked with West Ham's Robert Green but Foster was always amongst his leading targets.
The 28-year-old experienced had an outstanding first campaign at St Andrew's after a £6million move from Old Trafford.
He helped Blues to win the Carling Cup - their first trophy for nearly 50 years - but was powerless to stop them from being relegated from the Premier League.
Foster has made it clear he would prefer to remain in the midlands and a switch to Albion would fit the bill and maintain his desire to stay in the Premier League.
It is understood that cash-strapped Birmingham are keen to offload Foster's salary from their wage bill.
Hodgson also remains keen on signing Reading striker Shane Long to provide support for last season's 15-goal leading scorer Peter Odemwingie. But he faces competition from West Ham for the Republic of Ireland international.