De Ridder signed for Birmingham City on a free transfer on 3 July 2007. He made his Premier League debut on 12 August 2007 as a substitute in the opening match of the season against Chelsea, but injured his ankle within minutes of coming on and was unable to complete the match. Though starting the League Cup defeat at Blackburn Rovers, he was not considered fit enough for League games until after the October international break, when he made a substitute appearance against Manchester City. He was a key player in Birmingham's 3–2 win against Wigan Athletic on 27 October 2007 in which he made his full home debut.