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Abstract Background There is a critical need for the development of dependable and valid anxiety assessment tools suitable for people with moderate to severe intellectual disabilities, particularly those who speak few or no words. Distinguishing anxiety from distress caused by physical discomfort (pain) or characteristics associated with autism, prevalent in this population, necessitates specialised assessment tools. This study (a) developed a parent-report anxiety questionnaire tailored for individuals with severe to moderate intellectual disabilities, potentially with a co-diagnosis of autism, and (b) evaluated the psychometric attributes of this novel measure. Methods A comprehensive approach involving literature reviews, inspection of existing tools, and interviews with clinicians and parents guided the creation of the Clinical Anxiety Scale for People with Intellectual Disabilities. The tool was completed by parents or caregivers (N = 311) reporting on individuals aged 4 or older with intellectual disabilities. Results Exploratory factor analysis indicated a four-factor structure encompassing anxiety, pain, low energy/withdrawal, and consolability. The anxiety factor explained the most variance in scores (26.3%). The anxiety, pain, low energy/withdrawal subscales demonstrated robust internal consistency (α = 0.81-0.92), and convergent, divergent, and discriminant validity. Robustness of these subscales was further evidenced by test-retest reliability (ICC = 0.79-0.88) and inter-rater reliability (ICC = 0.64-0.71). Subgroup analyses consistently demonstrated strong psychometric properties among individuals diagnosed with non-syndromic autism (N = 98), children (N = 135), adults (N = 175), and across diverse communication abilities within the sample. Moreover, individuals diagnosed with both autism and anxiety exhibited significantly higher scores on the anxiety subscale compared to those without an anxiety diagnosis, while showing no difference in autism characteristic scores. Conclusions The findings indicate that the Clinical Anxiety Scale for People with Intellectual Disabilities is a promising measure for use across diverse diagnostic groups, varying communication abilities, and with people with moderate to severe intellectual disabilities.