Background: Owing to the high prevalence and detrimental consequences, postpartum depression (PPD) has been identified as one of the severe global public health issues in the last decade. Prior research found that during disasters or events, the prevalence rates of mental disorders among postpartum women are significantly high. However, the effect of the coronavirus disease 2019 (COVID-19) pandemic on PPD and its risk factors remained unclear for postpartum women. Therefore, the present systematic review and meta-analysis aimed to estimate the influence of the COVID-19 pandemic on the prevalence of PPD and to summarize risk factors for PPD during the COVID-19 pandemic. Methods: Three electronic databases of MEDLINE, EMBASE, and Cochrane library databases were systematically searched for articles from their commencements until 1 November 2021. Quality assessment of included studies, random-effects meta-analysis, and sensitivity analysis were performed. Results: A total of eight studies with 6480 postpartum women during the COVID-19 pandemic were included, and most studies were conducted in developed countries. The pooled prevalence of PPD was 34% (95% CI: 21–46%) during the COVID-19 pandemic, much higher than the incident of previous research during the non-pandemic period. Risk factors for PPD during the COVID-19 pandemic were defined as socio-demographic and clinical characteristics, stress and anxiety, lack of various supports, and the COVID-19 related factors. Conclusion: The research findings indicated that the COVID-19 pandemic could make detrimental effects on maternal mental wellbeing among women after childbirth. Investigating the prevalence and risk factors of PPD among postpartum women could shed some light on their mental and emotional states; so that support measures and tailored interventions from health professionals and policymakers could be offered to improve the maternal and infant outcomes, especially during the COVID-19 pandemic. Much more research on maternal psychological wellbeing during the COVID-19 pandemic was strongly recommended to undertake in the middle and low-income countries. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.