Purpose/Background The aim of this study is to examine the effects of memantine as an adjuvant treatment for obsessive compulsive (OC) symptoms in patients with bipolar disorder (BD) type I, manic phase. Methods/Procedures In this 16-week double-blind placebo-controlled randomized clinical trial, 58 patients in the manic phase of BD who had OC symptoms were randomly allocated to receive memantine or placebo plus their routine medications (lithium + olanzapine + clonazepam). The Yale Brown Obsessive Compulsive Behavior Scale was used to assess the outcomes. Adverse effects were also recorded. Findings/Results Thirty-eight patients (19 in the memantine group and 19 in the placebo group) completed the trial. Throughout the trial, the mean score decreased from 20.26 5.91 to 9.73 +/- 5.44 in the memantine group (P < 0.000) and from 22.89 +/- 5.70 to 16.63 +/- 4.00 in the placebo group (P < 0.000). At the end of the study, 15 (78.94%) patients in the memantine group and 7 (36.84%) patients in the placebo group demonstrated more than 34% decline in the Yale Brown Obsessive Compulsive Behavior Scale score (P < 0.01). No serious adverse effects were reported. Implications/Conclusions Our double-blind controlled clinical trial showed that memantine is an effective adjuvant agent for reducing OC symptoms in patients with BD. However, it needs to be noted that our study is preliminary, and larger double-blind controlled studies are needed to confirm the results.