Patients with major depressive disorder often experience relapse after responding to treatment; therefore, maintenance therapy with antidepressants is recommended for maintaining response or remission. This multicenter, open-label, flexible-dose, 52-week extension study evaluated the long-term safety, tolerability, and maintenance of efficacy in study participants who had completed one of two randomized, double-blind, placebo-controlled, 8-week dose-ranging vortioxetine trials in study participants with major depressive disorder. At the open-label baseline, all study participants were switched to vortioxetine 5 mg/day for the first week, with subsequent dose adjustments from 2.5 to 10 mg/day on the basis of response and tolerability. Treatment with vortioxetine for 52 weeks was well tolerated, with no new safety signals identified. Among the 834 evaluable study participants, treatment-emergent adverse events were reported in 70.6%, with the most common in the combined (all doses) population of nausea (15.2%), headache (12.4%), nasopharyngitis (9.8%), diarrhea (7.2%), and dizziness (6.8%). The rate of adverse events related to sexual dysfunction was low and weight gain was minimal. Laboratory values, vital signs, ECGs, physical examinations, and Columbia-Suicide Severity Rating Scale results showed no trends of clinical concern. The change in the severity of depressive and anxiety symptoms was maintained throughout the study as reflected by a 24-item Hamilton Depression Scale total score of 8.2 at week 52 (from 17.6 at open-label baseline) in the observed case data set.