There are 9 million prisoners worldwide, but it is uncertain what proportion have intellectual disabilities. Such prisoners have been identified as a vulnerable group at risk of victimization and mental illness. Psychiatric surveys based on interviews of unselected prison populations were sought, involving diagnoses of intellectual disabilities. The search was performed by computer-assisted searches, scanning of relevant reference lists, hand-searching of journals, and correspondence with authors of relevant reports. Prevalence rates of intellectual disabilities by gender and other potentially relevant study characteristics were abstracted from publications, supplemented by information from authors of reports. Ten relevant surveys from four different countries were identified that included a total of 11,969 prisoners (mean age: 29 years; 92% male; 23% violent offenders). No formal calculation of a summary estimate of prevalence was undertaken due to substantial heterogeneity. The results suggest that typically 0.5-1.5% of prisoners were diagnosed with intellectual disabilities (range 0% to 2.8% across studies). We conclude that the number of prisoners with intellectual disabilities is considerable and discuss implications for policy and practice.