Background: The COVID-19 pandemic caused major disruptions to maternity care, breastfeeding support, and social networks. These changes may have increased the risk of postpartum depression, anxiety, and stress among breastfeeding mothers, a population that has been underrepresented in previous reviews. This systematic review and meta-analysis aimed to compare maternal mental health outcomes among breastfeeding mothers before and during the COVID-19 pandemic. Methods: We searched MEDLINE, EMBASE, AMED, Web of Science, WanFang Data, MedRxiv, WHO COVID-19 databases, and grey literature from database inception to December 2023. Eligible studies compared mental health outcomes in breastfeeding mothers before and during the COVID-19 pandemic using validated assessment tools, including the Edinburgh Postnatal Depression Scale (EPDS), Generalized Anxiety Disorder Scale (GAD-7), State-Trait Anxiety Inventory (STAI), or Perceived Stress Scale (PSS). Studies with fewer than 10 participants per group were excluded. Two reviewers independently screened studies, extracted data, and assessed risk of bias using the Joanna Briggs Institute checklist or Newcastle-Ottawa Scale, depending on study design. Random-effects meta-analysis was performed when at least two studies reported comparable outcomes. Results: Twenty-three studies involving breastfeeding mothers from 15 countries were included. Meta-analysis showed significantly higher depressive symptoms during the pandemic compared with the pre-pandemic period, measured by EPDS (standardized mean difference [SMD] = 0.21, 95% confidence interval [CI] 0.14 to 0.29). Maternal anxiety measured by GAD-7 was also significantly higher during the pandemic (SMD = 0.27, 95% CI 0.13 to 0.41). Findings for perceived stress were mixed across studies and could not be pooled because of heterogeneity in reporting methods. Limited evidence suggested that mother-infant bonding did not substantially decline during the pandemic despite increased maternal psychological distress. Conclusions: Breastfeeding mothers experienced increased postpartum depression and anxiety symptoms during the COVID-19 pandemic. These findings highlight the importance of maintaining breastfeeding support services, ensuring access to maternal mental health screening, and developing flexible models of postpartum care during future public health emergencies. PROSPERO registration: CRD42022354670.