Background: To investigate the effects of social factors including social support and social restriction on postpartum depression (PPD) during the COVID-19 pandemic. Methods: We conducted a systematic review with PubMed and Web of Science from 1 January 2020 to 1 April 2023. Articles focusing on social factors and PPD during COVID-19 pandemic were investigated. Newcastle-Ottawa Scale and the Agency for Healthcare Research and Quality Scale were used to evaluate quality of literatures. Results: We included 31 articles (22 cross-sectional studies and 9 cohort studies) with good quality. Edinburgh Postnatal Depression Scale (EPDS) was used as the preferred measure for defining PPD. Social support reduced the prevalence of developing PPD by 30%–40%. Social restriction increased the risk of PPD but there was lack of evidence or effective scales to define restriction. Conclusions: We found that social support acted as an important protective factor for PPD during the COVID-19 pandemic and that social restriction increased the risk of PPD. EPDS may be an optimal choice for researchers to define PPD. Obstetricians should underline social factors for both pregnancy women and their families. The study was registered on PROSPERO (https://www.crd.york.ac.uk/prospero/), registration number: CRD42023434485.