Postpartum depression (PPD) is a common perinatal complication with adverse maternal and infant outcomes. This study investigated the efficacy and safety of zuranolone, a positive allosteric modulator of synaptic and extrasynaptic GABA receptors and neuroactive steroid, as an oral, once-daily, 14-day treatment course for patients with severe PPD. In this double-blind phase 3 trial, women with severe PPD were randomized in a 1:1 ratio to receive zuranolone 50 mg/day or placebo for 14 days. The primary endpoint was change from baseline in total score on the 17-item Hamilton Depression Rating Scale (HAM-D) at day 15; key secondary endpoints were change from baseline in HAM-D score at days 3, 28, and 45 and change from baseline in Clinical Global Impressions severity (CGI-S) score at day 15. Adverse events were monitored. Among 196 patients randomized (zuranolone, N=98; placebo, N=98), 170 (86.7%) completed the 45-day study. Treatment with zuranolone compared with placebo resulted in statistically significant improvement in depressive symptoms at day 15 (least squares mean LSM change from baseline in HAM-D score, -15.6 vs. -11.6; LSM difference, -4.0, 95% CI=-6.3, -1.7); significant improvement in depressive symptoms was also reported at days 3, 28, and 45. CGI-S score at day 15 significantly improved with zuranolone compared with placebo. The most common adverse events (≥10%) with zuranolone were somnolence, dizziness, and sedation. No loss of consciousness, withdrawal symptoms, or increased suicidal ideation or behavior were observed. In this trial, zuranolone demonstrated significant improvements in depressive symptoms and was generally well tolerated, supporting the potential of zuranolone as a novel, rapid-acting oral treatment for PPD.