Background: Individuals with Down syndrome (DS) experience health disparities possibly associated with high levels of sedentary behaviour (SB). We systematically reviewed SB measurement, levels, patterns, correlates, consequences and interventions in people with DS across the lifespan. Method: We searched these databases: Embase; PubMed; Web of Science; Scopus; CINAHL; PsycINFO; SPORTDiscus; and Cochrane Library. We included quantitative studies published in English since 1 January 1990. Results: We identified 17 eligible articles. Across studies, accelerometer-determined sedentary time was 552 min/day (median: 542; range 392-680 min/day). Sedentary time appeared higher than previously reported values in the general population, especially for youth with DS. Multivariate SB correlates were primarily familial and environmental. Sedentary behaviour consequences have not been exclusively studied. Motor skills training reduced SB in youth with DS. Conclusions: Sedentary behaviour research in this population is in early stages. Individuals with DS have high levels of SB that may be responsive to interventions.