This double-blind placebo-controlled, fixed/flexible-dose phase 2 trial assessed the efficacy, safety, and tolerability of cariprazine vs. placebo for depressive episodes associated with bipolar I or II disorder. Primary endpoint was change in Montgomery-Åsberg Depression Rating Scale (MADRS) total scores (baseline to week 8), and secondary endpoint was mean Clinical Global Impressions-Improvement score (week 8). Patients were randomized (N = 233) 1:1:1 to placebo, 'low-dose' 0.25-0.5 mg/day or 'high-dose' 1.5-3.0 mg/day cariprazine. Adverse events, laboratory results, vital signs, extrapyramidal symptoms, and suicide risk were monitored. Neither cariprazine group significantly separated from placebo in primary (mixed-effect model repeated measures MADRS least-squares mean differences: low-dose = -0.7, P = 0.7408; high-dose = 0.0, P = 0.9961) or secondary efficacy measures. No new safety signals with cariprazine were observed and common treatment-emergent adverse events (≥5% of cariprazine patients and twice the rate of placebo) included insomnia, akathisia, dry mouth, nausea, weight increased, diarrhea, restlessness, vomiting, musculoskeletal stiffness, migraine, and cough. Metabolic and weight changes were generally similar for cariprazine and placebo. Factors that may have affected the outcome of the trial were identified, which helped to inform the design and conduct of subsequent phase 2b/3 clinical trials of cariprazine in bipolar depression.