OBJECTIVE: Several randomized controlled trials (RCTs) investigated omega-3 polyunsaturated fatty acids (PUFAs) (ie, fish oil) in perinatal depression, but their efficacy remains unclear. We performed a meta-analysis of RCTs on omega-3 PUFAs for perinatal depression, comparing a priori defined subgroups: pregnant women vs postpartum women and prevention vs treatment of perinatal depression. METHODS: We searched Web of Science, Embase, PsycINFO, and the Cochrane Library, combining omega-3 PUFAs and perinatal depression terms and including publications up to February 18, 2019, for RCTs on omega-3 PUFAs compared to placebo or any active comparator. RESULTS: Data from 18 RCTs on 4,052 participants showed an overall significant small beneficial effect of omega-3 PUFAs on depressive symptoms compared to placebo (-0.236 standardized difference in means [SDM]; 95% CI = -0.463 to -0.009; P = .042). Heterogeneity was considerable (I² = 88.58; P < .001), with significant subgroup differences explaining 55% of between-study variance (P = .001). In depressed women, omega-3 PUFAs showed a medium effect (SDM = -0.545; 95% CI = -1.182 to 0.093; P = .094) vs no effect in nondepressed women (SDM = -0.073). Moreover, the effect was medium to large in postpartum women (SDM = -0.656; 95% CI = -1.690 to 0.378; P = .214) compared to a negligible effect during pregnancy (SDM = -0.071). RCTs specifically studying postpartum depression showed the largest effect (SDM = -0.886; 95% CI = -2.088 to 0.316; P = .149). CONCLUSIONS: Omega-3 PUFAs have an overall significant small beneficial effect on perinatal depression, with important subgroup differences. We advise against prescribing omega-3 PUFAs for the treatment or prevention of depressive symptoms during pregnancy, given a lack of effect with low heterogeneity. In contrast, omega-3 PUFA supplementation may be a promising (add-on) treatment for postpartum depression.