Individuals with eating disorders (EDs) may be particularly susceptible to body image related cognitive fusion (i.e., excessive entanglement with one's body image related thoughts such that they unduly influence on behavior). The Cognitive Fusion Questionnaire-Body Image (CFQ-BI) is the only existing measure of this construct, yet its psychometric properties have not been examined within a clinically diagnosed ED sample. The current study used confirmatory factor analysis and explored measurement invariance, construct validity, and incremental validity of the CFQ-BI when used with adolescent (n = 75) and adult (n = 100) females admitting to residential ED treatment. A modified version of the single-factor structure of the CFQ-BI best fit the data and configural, metric, and scalar invariance were supported across age groups (i.e., adolescents or adults) and ED behavioral presentations (i.e., restrictive behaviors or binge/purge behaviors). Adults' and individuals with binge/purge behavioral presentations reported significantly more body image related fusion compared to adolescents and adults, respectively. Body image related psychological flexibility and ED severity were both significantly correlated CFQ-BI scores in expected directions. These findings suggest the CFQ-BI is a valid measure for assessing body image related fusion among adolescent and adult females with varying ED behavioral presentations. Implications and future research directions are discussed.