Background: Adults with intellectual disabilities are reported to be highly inactive, with research required to understand contributory factors. This systematic review aimed to investigate gender differences in physical activity (PA) and sedentary behaviour (SB) in adults with intellectual disabilities. Methods: This systematic review was reported in accordance with PRISMA guidelines. Seven databases were searched up to, and including, January 2018. Screening identified papers that assessed gender-specific PA and/or SB outcomes in adults with intellectual disabilities. Data were synthesized using a narrative synthesis and random effects model meta-analyses. Results: Twenty-six papers were included; 25 measured PA, and eight assessed SB. Women with intellectual disabilities were least active with a significant overall effect of gender identified. For SB, no consistent gender differences were found. Conclusions: Reflecting the general population, men with intellectual disabilities were most active. Intellectual disability research should consider the role of gender to inform future interventions targeting inactivity.