The aim of this study was to investigate the efficacy and safety of ondansetron as an augmentative agent to fluvoxamine in the treatment of patients with obsessive–compulsive disorder (OCD). Forty-six men and women, aged 18–60 years, who fulfilled the diagnostic criteria of OCD on the basis of the DSM-IV-TR and had a Yale–Brown Obsessive Compulsive Scale (Y-BOCS) score of at least 21 were recruited into the study. The patients randomly received either ondansetron (8 mg/day) or placebo for 8 weeks. All patients received fluvoxamine (100 mg/day) for the first 4 weeks, followed by 200 mg/day for the rest of the trial. The patients were assessed using the Y-BOCS and the adverse event checklists at baseline, and the second, fourth, sixth, and eighth week. Forty-four patients completed the study. The Y-BOCS total score as well as the Y-BOCS obsession subscale score and compulsion subscale score showed significantly greater reduction in the ondansetron group than in the placebo group. There was no significant difference in adverse events between the two groups. In this 8-week double-blind randomized-controlled trial, ondansetron showed significant beneficial effect as an augmentative agent with fluvoxamine in patients with moderate to severe OCD and it was generally well tolerated.