Suicide is a leading cause of death in prison populations worldwide. In addition to suicide deaths, suicidal ideation and non-fatal suicide attempts also warrant attention. Because suicidal ideation is a significant predictor of subsequent (attempted) suicide, as well as important in its own right as an indicator of emotional distress, the improved understanding of the prevalence and correlates of suicidal ideation in prisoners is critical in order to inform suicide prevention efforts. Between October 2015 and May 2016, a cross-sectional study was conducted in fifteen prisons in Flanders, Belgium. Participants were a random sample of 1,326 prisoners who completed a survey, covering a wide range of relevant sociodemographic, criminological, psychiatric and institutional variables. Results are compared to international findings, and policy implications and recommendations are discussed.