© 2016. Background: Obsessive-compulsive disorder (OCD) is a chronic psychiatric disorder that is causally linked to dysregulation of the serotonergic system. The aim of this study is to investigate the efficacy of Withania somnifera (W. somnifera) root extract as an adjunct therapy to standard OCD treatment. Methods: Thirty patients with a confirmed diagnosis of OCD according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) criteria participated in this randomized double-blind placebo-controlled trial and were randomly assigned to the treatment group (W. somnifera extract, 120 mg/day; n = 15) or the placebo group (n = 15). All patients were under treatment with Selective Serotonin Re-uptake Inhibitors (SSRIs), and were instructed to take 4 capsules of the extract or placebo per day, preferably after meals, for a period of six weeks. The Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) was used in order to assess the severity of OCD symptoms at baseline and at the end of the trial. Statistical analyses were performed using SPSS software and Y-BOCS values were presented as median and range (Min-Max). Results: Comparison of the change in Y-BOCS score during the course of the trial revealed a significantly greater effect of W. somnifera (26 (14-40) [pre-treatment] versus 14 (4-40) [post-treatment]; change: -8 (-23 to 0)) versus placebo (18 (11-33) [pre-treatment] versus 16 (10-31) [post-treatment]; change: -2 (-4 to 0)) (P <0.001). The extract was safe and no adverse event was reported during the trial. Conclusion: W. somnifera extract may be beneficial as a safe and effective adjunct to SSRIs in the treatment of OCD.