Cognitive fusion--or the tendency to buy into the literal meaning of thoughts, feelings, and bodily sensations--plays an important role in the etiology and maintenance of anxiety disorders and figures prominently in third-generation behavior therapies such as acceptance and commitment therapy (ACT). Nonetheless, there is a lack of validated self-report measures of cognitive fusion/defusion, particularly in the area of anxiety disorders. We attempted to fill this gap with the development and validation of a self-report cognitive fusion measure, the Believability of Anxious Feelings and Thoughts Questionnaire (BAFT), in both a healthy undergraduate sample (N = 432) and highly anxious community sample (N = 503) undergoing a 12-week online ACT intervention. Results suggested a hierarchical factor structure of the BAFT with three lower order factors and one hierarchical factor and excellent internal consistency for the total BAFT score (i.e., [alpha]S = 0.90 and 0.91 for the undergraduate and highly anxious samples, respectively) and for its factors. Additionally, the BAFT and all of its factors consistently showed strong construct validity with other relevant process and outcome measures in both samples, strong 12-week test-retest reliability (r = 0.77) in our highly anxious waitlist control subsample and responsiveness to treatment in our highly anxious intervention subsample. These findings suggest that the BAFT is a reliable and valid measure of cognitive fusion in both healthy and clinical populations. (Contains 6 tables.)