Midfielder Craig Gardner has completed his move from Albion to Blues in a deal which will become permanent in the summer.
The rival clubs have agreed an initial loan deal in addition to a fee which will see Gardner move to St Andrews permanently at the end of the season.
Baggies head coach Tony Pulis said: “Craig has done a great job for me and the Club and we were certainly not in any hurry to see him leave.
“But he is desperate to play first team football which I could not guarantee and this is a terrific opportunity for him at Birmingham City. He goes with our very best wishes.”
The 30-year-old made 85 appearances in two-and-a-half seasons for the Baggies, after joining from Sunderland in the summer of 2014.
He previously played for Blues, his boyhood club, between 2010 and 2011, winning the League Cup before leaving following the club’s relegation from the Premier League.Subscribe to our Newsletter