The Wales international right-back has agreed to join Lee Clark's side on a three-year deal which will begin on July 1.
"I'm over the moon," Eardley exclusively told bcfc.com shortly after agreeing the deal. "To be signing for a club of this stature is the next step in my career and I'm hoping to get some games under my belt for a very big club and kick on.
"The opportunity came about and there were discussions between myself and those at the club and I was just glad that we could get something agreed. I would have never turned down an opportunity to be signing for such a big club.
"I know from my time at Blackpool with Barry Ferguson and Kevin Phillips, as they touched on it a good few times, how big a club Birmingham is. I've played at St. Andrew's myself and the fans take the roof off. Everybody I have spoken to only has praise for the club.
"Lee Clark touched on a few things about me personally, what he would like to do next season and where he'd like to take this football club, it just totally made my mind up. After those chats, it was an easy decision."