<p>Family accommodation (FA) is defined as any behavior on the part of family members, including parents and caregivers, that functions to facilitate maladaptive anxiety-related avoidance in youth. Due to its role in perpetuating cycles of negative reinforcement, FA is a key mechanism underpinning youth anxiety as well as a critical treatment target. The Family Accommodation Scale-Anxiety (FASA) is a parent-report questionnaire designed to capture facets of FA involving child anxiety-related modification of familial routines and participation in child anxiety-driven behaviors. In the current study, we psychometrically examined the FASA using a multi-study sample (<i>k</i> = 4 studies, <i>N</i> = 379 total) of treatment-seeking anxious youth and their caregivers. Results of confirmatory factor analysis indicated that a two-factor model (with Participation and Modification subscales) fit the data well. The FASA also demonstrated evidence of optimal internal consistency. Findings of convergent validity revealed theory-consistent correlations between the FASA and measures of youth anxiety severity, autism features, externalizing symptomology, and caregiver-reported strain. The FASA also demonstrated measurement invariance across sex-at-birth (male vs. female), age (12 or younger vs. 13 or older), ethnicity (Hispanic vs. non-Hispanic), and presence (vs. absence) of autism. Evidence of treatment sensitivity was also provided via theory-consistent changes in FA from pre- to post-treatment. Our results indicate that the FASA is a psychometrically sound measure for assessing FA in clinical settings with youth representing a range of diversity factors. Findings are discussed in the context of continuing to improve precision in measurement of FA for anxious youth and their caregivers.</p>