Background: Sedentary behaviour (SB) among adults with Down syndrome (DS) may differ based on personal or environmental factors. Objective: Investigate differences in SB levels and patterns of adults with DS based on sex, age, and residence-type across weekdays and weekends. Methods: Thirty-four adults with DS (15 men; 37 ± 12 years) underwent accelerometry-based measurements of sedentary time, bouts, and breaks for 7 days. We evaluated differences with 2 × 2 mixed-model (group-by-day) ANOVA. Results: Younger (19-36 years) individuals had less sedentary time (p = 0.042), and shorter (p = 0.048) and fewer (p = 0.012) bouts than older (37-60 years) individuals. Group home residents had more bouts on weekends than adults living with parent/guardians (p = 0.015). Conclusions: Adults with DS spent half their waking hours in SB of short bouts. Age and residence may influence SB.