Thought-action fusion (TAF) is a cognitive error that has been frequently investigated within the context of obsessive-compulsive disorder (OCD). However, evidence suggests that this error may also be present in disorders other than OCD, indicating that TAF is related to higher order factors rather than a specific diagnosis. We explored TAF in a sample of patients with mixed diagnoses undergoing treatment with a transdiagnostic CBT protocol. Elevated TAF levels at baseline were not specific to patients with OCD. However, the presence of any generalized anxiety disorder (GAD) diagnosis was unexpectedly the strongest predictor of likelihood TAF. Likelihood TAF, a particular component of TAF, was reduced after transdiagnostic treatment, and this reduction was not affected by the presence of a GAD diagnosis. Results indicate that TAF is responsive to treatment and should be assessed and, perhaps, treated in disorders beyond OCD.