The 26-year-old Belgian left the Saints by mutual consent after spending the last six months of the season at Bolton Wanderers.
"FC Utrecht is a nice club, with an avid and passionate supporters," De Ridder told the club's official website.
"I hope to be of great value for FC Utrecht. Here I am going to do my best."
De Ridder, who arrived at St. Mary's from De Graafschap in 2011, has signed a three-year deal at the Stadion Galgenwaard.