Supporting positive change in lifestyle behaviours is a priority in tackling the health inequalities experienced by adults with intellectual disabilities. In this systematic review, we examine the evidence on the definition, measurement and epidemiology of sedentary behaviour of adults with intellectual disabilities. A systematic literature search of PUBMED, EMBASE, MEDLINE and Google Scholar was performed to identify studies published from 1990 up to October 2015. Nineteen papers met the criteria for inclusion in the systematic review. Many researchers do not distinguish between insufficient physical activity and sedentary behaviour. None of the studies reported the reliability and validity of the methods used to measure sedentary behaviour. Sedentary time, assessed objectively, ranged from 522 to 643 min/day: higher than in adults without intellectual disabilities. This first-ever review of sedentary behaviour and intellectual disabilities found that at present the evidence base is weak. Studies calibrating accelerometer data with criterion measures for sedentary behaviour are needed to determine specific cut-off points to measure sedentary behaviour in adults with intellectual disabilities. Researchers should also examine the reliability and validity of using proxy-report questionnaires to measure sedentary behaviour in this group. A better understanding of sedentary behaviour will inform the design of novel interventions to change lifestyle behaviours of adults with intellectual disabilities.