The former Aston Villa man was interesting the Baggies in the summer and had a transfer request turned down, but he finally has the move he wanted after passing his medical
The 27-year-old has signed a two-and-a-half-year deal at the Hawthorns but is ineligible to face Fulham in the Baggies' next Premier League fixture, to be played on Wednesday.
The Englishman joins his third Midlands club after a previous spell with Aston Villa and having spent four-and-a-half seasons at St Andrew's.
Ridgewell will take the number six shirt and joins a West Brom side currently 15th in the Premier League table.
Roy Hodgson's men have conceded 31 goals in 22 games this season and will hope that Ridgewell can improve that record.
The defender becomes part of the continuing exodus of players from Birmingham since last season's relegation from the Premier League.