Background: Trait anxiety, a person's general tendency to experience anxiety, has been widely researched in neurotypical populations resulting in standardised measures being well established for reliability and validity. However, the psychometrics of these measures have rarely been explored for children on the autism spectrum. Method: This study used an online questionnaire to investigate the item response profile and psychometric properties of the State-Trait Anxiety Inventory for Children – Parent reported Trait subscale (STAIC-P-T) in a sample of 105 parents of children with an autism diagnosis (mean age 10y5m; SD 2y10m). Results: STAIC-P-T items with the highest mean score related to the child's feelings of worry or upset. Items related to physiological symptoms had the lowest mean score. The STAIC-P-T showed good internal consistency (α = 0.84), evidence of divergent validity through very weak correlation with a measure of autism characteristics (rs =.06, p = .58), and convergent validity through strong positive correlation with an established measure of anxiety (rs =.65, p < .001). However, confirmatory factor analysis did not support the unidimensional model of trait anxiety established in the studies with neurotypical participants. Conclusions: Despite evidence supporting the validity and reliability of the STAIC-P-T, factor analysis indicates the need for further research to explore the suitability of this measure to assess trait anxiety in autism.