Objective: The present study aimed to investigate whether the dysfunctional obsessive beliefs are specific to obsessive-compulsive disorder (OCD) using three different clinical groups including OCD, depression, and anxiety and a control group of university students. Method: The participants of the study comprised three patient groups with OCD (n=53), major depressive disorder (MDD, n=67), anxiety disorders (AD, n=73), and a group of university students (n=477). The short version of the Obsessive Belief Questionnaire (OBQ-20) was used to measure obsessive beliefs. The Semi-Structured Clinical Interviews for DSM-IV-TR (SCID-I), the Obsessive-Compulsive Inventory Revised Form, the Beck Depression Inventory, and the State-Trait Anxiety Inventory-Trait Form were used to assess the severity of the symptoms. Results: The scores of the three patient groups were significantly higher on the OBQ-20 as compared to the university students. It is noteworthy that the OBQ-20 scores did not significantly differ between the three patient groups except on the 'importance of thought' (ICT) subscale showing significantly higher scores in the group with OCD in comparison to the patients with depression and anxiety disorders. Conclusion: The results suggest that obsessive beliefs may have a transdiagnostic mechanism with a possible role in the etiology and maintenance of a broad range of different psychopathologies, except in relation to the 'importance and control' of thoughts. Investigating the common processes underlying different psychopathologies is important for the etiological explanation and future treatment of the disorders.