This study aimed to quantify and compare sedentary behaviour patterns in patients with severe mental illness as stratified by gender, age, body mass index, distress, illness duration and antipsychotic medication using both objective and self-reported methods. Sedentary behaviour patterns were measured in 90 outpatients (mean age±SD: 41.6±9.2 years, 20% women) with severe mental illness (primarily schizophrenia, n=63) using the SenseWear Armband and the Sedentary Behaviour Questionnaire. They spent 58% of waking time sedentary, primarily watching television. Differences between methods were not significant for the overall group or for stratified groups. Both methods showed significant correlation for weekday for the overall group. According to the stratified groups, youngers showed a significant correlation for weekday and average day sedentary time, and the high illness duration and low antipsychotic medication groups for weekday. Significant differences in sedentary behaviours between stratified groups were only detected with the SenseWear. Patients with severe mental illness had high levels of sedentary behaviours, with watching television being the most prevalently reported. We found a low validity in the self-reported estimates of sedentary time by this population, being higher on weekdays for the overall group and for the younger, high illness duration and low antipsychotic medication groups.