Objectives The aim of this study was to assess levels and patterns of physical activity and sedentary behaviour among inpatient adults with mental illness.

Design Cross-sectional.

Methods 101 participants completed questionnaires on time spent in walking, moderate- and vigorous-intensity activity in the past week and domain specific sitting time on a usual weekday and weekend day. 36 participants also provided valid accelerometry data. Regression analyses were used to explore associations between MVPA and sedentary behaviour and explanatory variables of gender, age, education, body mass index and psychological distress.

Results Self-report data indicated median of 32 min/day (IQR: 14.46–85.71) in weighted MVPA and a median of 761 min/day (12.7 h) (IQR: 552.43–917.14) in sedentary behaviour. Accelerometry data indicated an average of 115 min/day in light activity, 37 min/day in MVPA and 664 min/day (11.1 h) in sedentary behaviour. Bivariate analyses indicated no significant associations between explanatory variables and MVPA and sedentary behaviour.

Conclusions Inpatient adults with mental illness can be physically active, with walking comprising the major component of MVPA time. Inpatient adults with mental illness spend a significant amount of time sitting; intervention strategies could focus on reducing the time spent sitting in general relaxation and doing nothing.