Introduction and Objective. Despite the effectiveness of electroconvulsive therapy (ECT) in treating major depressive disorder (MDD), its cognitive side effects make it less popular. This study investigated the impact of liothyronine on ECT-induced memory deficit in patients with MDD. Methodology. This is a double-blind clinical trial, in which 60 patients with MDD who were referred for ECT were selected. The diagnosis was based on the criteria of DSM-IV-TR. Patients were divided randomly into two groups to receive either liothyronine (50 mcg every morning) or placebo. After the assessment with Wechsler Memory Scale-Revised (WMS-R) before first session of ECT, posttests were repeated again, two months after the completion of ECT. Findings. By controlling the pretest scores, the mean scores of the experimental group were higher than the control group in delayed recall, verbal memory, visual memory, general memory, and attention/concentration scales (P<0.05). Conclusion. Liothyronine may prevent ECT-induced memory impairment in patients with MDD. This study has been registered in IRCT under IRCT201401122660N2.