The absence of assessment tools incorporating different cultural models of mental illness is a major barrier to recruiting ethnic minorities into clinical trials, reducing generalisability of findings and potentially increasing disparities in access to evidence-based care. This study aimed to develop and validate a new Knowledge about Psychosis (KAP) self-report measure and a culturally-adapted version for African-Caribbean people (CaKAP). Content and face validity were achieved through consultations with experts in psychosis and a focus group with service users, carers, and community members. Eighty-seven predominantly White British participants and 79 African-Caribbean participants completed the knowledge questionnaires (KAP and CaKAP) and measures of help-seeking and stigma. Overall, the measures showed good internal consistency and test re-test reliability. Construct validity was evidenced via significant positive associations between knowledge about psychosis and help-seeking and significant negative associations between knowledge and stigma. These measures could improve the delivery of psychosocial interventions and outcome measurement in research trials.