Background: Adherence to antiretroviral therapy (ART) is crucial for preventing vertical transmission of HIV and maternal deaths. While the literature is replete with studies on ART adherence, the role of social capital in adherence to ART is less studied. Drawing from the social cognitive theory, this study examines the relationship between social capital and post-partum adherence to ART. Methods: We analysed data from 481 post-partum women with HIV in the Eastern Cape, South Africa. Adherence to ART was measured using a validated scale, capturing self-reported medication-taking behaviours. Social capital was assessed using a pre-validated tool, encompassing indicators of social engagement, support networks and community connectedness. We used logistic regression models to examine social capital and ART adherence associations, while controlling for relevant covariates. Results: Participants mean age was 32.9 (SD ± 5.76) years. After adjusting for age, education level, alcohol use, status disclosure to partner, marital status, desire for more children, employment status and living arrangements, social capital was significantly associated with higher odds of ART adherence (p = 0.004, AOR 1.09; 95% CI 1.03–1.16). Conclusion: We found evidence in support of the role of social capital in ART adherence. Strengthening social support networks and addressing psychosocial factors could improve adherence to ART. As such, policymakers and programme managers should consider the role of social capital in designing interventions to improve ART adherence. Future research should explore the mechanisms through which social capital impacts ART adherence.