It remains unclear how the COVID-19 pandemic has affected the mother-infant relationship and associations between maternal postpartum depression (PPD) and offspring temperament. This study examined the impact of the pandemic on these links and how maternal ratings of the mother-infant relationship mediated associations between PPD and infant temperament in a sample of treatment-seeking mothers in Ontario, Canada before and during the COVID-19 pandemic. Mothers with infants <12 months of age and Edinburgh Postnatal Depression Scale scores ≥10 enrolled in two separate randomized controlled trials of 1-day cognitive behavioral therapy-based workshops for PPD conducted before COVID-19 ( = 392) and during the pandemic ( = 403). Mothers reported on depressive symptomatology, infant temperament, and the mother-infant relationship. Maternal PPD was associated with more infant negative affectivity and mother-infant relationship difficulties. While associations between PPD and infant-focused anxiety were stronger during COVID-19, the pandemic did not otherwise affect associations between PPD and infant temperament. Mediation analyses suggested that aspects of the mother-infant relationship mediated associations between PPD and infant negative affectivity. Findings highlight the importance of detecting PPD and intervening to potentially improve outcomes for mothers and their children.