Background Substantially elevated levels of stress, depression and anxiety have been reported by individuals that were pregnant and in the early postpartum throughout the COVID- 19 pandemic (Khoury et al., 2023). As perinatal mental health can have lasting impacts on parent health and child development, identifying protective factors to mitigate negative outcomes is crucial. This study investigated whether relationship resilience buffers the association between perceived stress during pregnancy and postpartum depression, up to 15 months postpartum. Method This longitudinal study followed 304 participants, from Ontario, Canada, during pregnancy, 6 weeks, 6 months, and 15 months postpartum. During pregnancy, perceived stress was measured using the Perceived Stress Scale and a latent “relationship resilience” factor was estimated using measures of social support, martial conflict (reverse scored) and relationship closeness. Depression was measured at all timepoints using the Center for Epidemiologic Studies Depression Scale. Structural equation models were conducted using MPLUS to examine the main effects and interaction of pregnancy stress and relationship resilience on postpartum depression. Results After controlling for significant covariates, relationship resilience buffered the impact of perceived stress during pregnancy on postpartum depressive symptoms at 15 months postpartum (β = -0.158, SE= 0.073, 95% CIs: -0.302, -0.014), but not at 6 weeks or 6 months postpartum, identifying a possible “sleeper effect”. Conclusion Results indicate longitudinal effects of pregnancy stress and resilience on postpartum mental health. Findings underscore the importance of continued investigation of long-term impacts of the COVID-19 pandemic on families, as this is a public health priority.