Rationale: Quetiapine extended release (XR) has been used to treat various psychiatric disorders, including depressive episodes associated with bipolar I and II disorders. Quetiapine XR is the first approved drug in China for the treatment of bipolar disorder. Objectives: The study evaluated the efficacy and safety of short-term quetiapine XR monotherapy in the treatment of depressive episodes of bipolar I and II disorders. Methods: This was an 8-week multi-center, randomized, double-blind, placebo-controlled, fixed-dose phase 3 study. The primary endpoint was the mean change of the Montgomery-Åsberg Depression Rating Scale (MADRS) total score. Secondary endpoints included Clinical Global Impressions-Bipolar (CGI-BP) and remission rates. Results: The study recruited 279 adult bipolar I or II patients currently experiencing depression from 11 Chinese provinces. Of these, 139 received quetiapine XR (300 mg/day) and 140 received placebo for 8 weeks. The mean change in the MADRS total score was significantly greater in the quetiapine XR group than in the placebo group (-19.00 ± 7.88 vs. -16.20 ± 9.32; p = 0.004). Adverse events occurred in 96 patients (65.3 %) in the quetiapine XR group and 72 (49.0 %) in the placebo group. The incidence of serious adverse events did not differ significantly between the groups (p = 0.247). Conclusions: This study, which is the first to evaluate 300 mg/day quetiapine XR monotherapy for depression in Chinese patients with bipolar disorders, found that this drug was superior to the placebo. Quetiapine XR was generally safe and well tolerated (ClinicalTrials.gov number, NCT01256177). © 2016 Springer-Verlag Berlin Heidelberg.