<b><i>Background:</i></b> Preliminary evidence is promising regarding the anxiolytic effects of statins in animal models of anxiety. Hence, this study aimed to evaluate the efficacy of simvastatin augmentation versus placebo in the treatment of patients with generalized anxiety disorder (GAD) with residual symptoms despite treatment with selective serotonin reuptake inhibitors (SSRIs). <b><i>Methods:</i></b> A double-blind, 8-week controlled trial was conducted from August 2018 to December 2019 in an outpatient psychiatry clinic in Hamadan, Iran. A total of 138 patients with a diagnosis of GAD were assessed for eligibility. Of them, 84 patients who met the study criteria were randomly assigned either to the adjuvant simvastatin (20 mg/day) or to the placebo group. Standard medication consisting of SSRIs was consistent 2 months prior to and during the study. The severity of anxiety symptoms for each patient was assessed based on the Hamilton Anxiety Rating Scale (HAM-A) score at baseline, week 4, and week 8 after treatment. Additionally, blood lipid values were assessed at baseline and on completion of the study. <b><i>Results:</i></b> Thirty-three out of 42 patients in the intervention group and 35 out of 42 patients in the control group completed the 8 weeks of the study period. Compared to the placebo group, in the simvastatin group cholesterol, triglycerides, and low-density lipoprotein significantly decreased, and high-density lipoprotein significantly increased over time. General linear model analysis demonstrated that although over time a higher decrease in mean HAM-A scores was observed in the intervention group compared to the control group, this difference was not statistically significant (<i>p</i> = 0.11). In addition, at the end of the study, the number of responders and remitters was comparable in the two groups. <b><i>Conclusions:</i></b> The results from this clinical study did not support the potential efficacy of adjunctive simvastatin in the treatment of patients with GAD. Thus, large-scale and long-term clinical trials are required to more accurately assess the potential efficacy of statins in the treatment of patients with anxiety disorders.