BACKGROUND: Better interventions are needed to help individuals diagnosed with psychosis to be more physically active and less sedentary. Cognitions are likely to be crucial in blocking and initiating movement. Our aim was to identify cognitions likely to affect physical inactivity or sedentary behaviour in patients with psychosis in order to inform future research and intervention development. METHODS: Item pools of 149 cognitions related to physical activity (46 promoting and 103 blocking) and 156 cognitions related to sedentary behaviour (105 promoting and 51 blocking) were developed with lived experience advisors and administered separately to patients with diagnoses of non-affective psychosis, aged 16 years or older, accessing secondary care mental health services in 43 National Health Service (NHS) providers in England and Wales. The study period was between 6th March 2023 and 19th February 2024. Measures were developed using exploratory and confirmatory factor analysis. Internal reliability and test-retest reliability were assessed. The International Physical Activity Questionnaire-Short form was used to validate the physical activity cognitions measures and the Sedentary Behaviour Questionnaire was used to validate the sedentary behaviour cognitions measure. FINDINGS: Nine hundred and seventeen participants with non-affective psychosis were enrolled. 563 patients completed the physical activity item pools and 523 patients completed the sedentary behaviour item pools, with 169 patients completing both questionnaires. 620 (67.6%) patients were men and 286 (31.2%) patients were women. 592 (64.6%) patients were White, 83 (9.1%) were mixed or multiple ethnic origin, 85 (9.3%) were Asian, 133 (14.5%) were Black, 4 (0.4%) were Arab, and 20 (2.2%) were other ethnic groups or preferred not to say. The mean age of the cohort was 42.9 (12.9 [18-81]) years.The Oxford Physical Activity Cognition (OPAC) (Promoting and Blocking) scales comprised eight factors: positive feelings with exercise, anticipation of reward, responsibility to others, negative social appraisal, physical health, hearing voices, preoccupation, and negative feelings towards exercise. The Oxford Sedentary Cognitions Assessments (OSCA) (Promoting and Blocking) comprised 11 factors: purpose, poor physical health, defeatist beliefs, unaware of time passing, low mood, television, sitting to steady mind, purpose, anticipation of reward, responsibility to others, and motivation from others.All four measures had excellent model fits. Internal reliability and test-retest reliability were good. The measures had small associations with self-reported physical activity and sedentary behaviour. INTERPRETATION: The cognitive drivers of movement for patients with psychosis appear numerous. There are many different types of content of cognitions that could hinder or facilitate movement in patients diagnosed with psychosis, with a number overlapping across physical activity and sedentary behaviour and a number specific to each. These cognitions could be future plausible targets for interventions to increase movement. Required now is causal evidence of whether these cognitions are amenable to change and, if they are, whether this leads to behavioural change. Further research is also needed to replicate the findings. FUNDING: National Institute for Health and Care Research.