Ridgewell has put pen to paper on an initial two-and-a-half year contract plus the option of a further 12 months in the club's favour.
The 27-year-old successfully passed a medical at the Baggies training ground on Tuesday afternoon after agreeing personal terms.
Ridgewell signed too late to be eligible for Wednesday evening's Premier League game at Fulham but will come into contention to make his debut in Saturday's home encounter with Swansea.
West Brom head coach Roy Hodgson said: "Liam is a very experienced defender and has the added benefit of being left-sided. We do not have many of those defenders at the club.
"He can also play in two positions (left-back and centre-back) and he is used to playing in teams under pressure to get results to stay in the league.
"We hope we won't be one of those teams under pressure but, if we are, it is good to have someone of that experience.
"Liam was a transfer target last summer and since the start of this January but negotiations have been long and arduous which is often the case."
Ridgewell submitted a transfer request during the summer and made it clear his desire to leave Birmingham had not altered.
The move brings to an end Ridgewell's four-and-a-half-year spell at St Andrew's and he will link up again with goalkeeper Ben Foster who is on loan from Birmingham to West Brom.
