In rural Malawi, adolescent mothers represent 31% of pregnancies. While some adolescent mothers experience motherhood as an exciting, positive, and affirming experience, for others, it may increase their risk of postnatal depression (PND). Social norms and culture contribute to adolescent mothers’ experiences and may influence their mental health. However, there is limited research around the prevalence of PND and its cultural influences among adolescent mothers. A cross-sectional survey was administered from 7thSeptember 2021 to 31stMarch 2022. Three hundred and ninety-five adolescent postnatal mothers aged ≤19 were conveniently recruited at Mitundu hospital in Lilongwe, Malawi. The Edinburgh Postnatal Depression Scale was used to assess depression. A cutoff point of ≥ 10 was employed to categorize probable PND. Binary logistic regression was used to determine the predictors of depression. The mean age of participants was 17 (SD 1.157). and 43.6% (n = 172) presented with PND (EPDS scores ≥10). When all factors were considered in a binary logistic model, adolescents who had ever experienced intimate partner violence (IPV) were 13.6 times more likely to report PND after controlling for age and other predictors compared to those without an experience of IPV (aOR 13.6, p = 0.01, 95% CI 2.10–88.9). Participants whose families did not decide for them (regarding their care) were 2.3 times more likely to present with PND than those whose families decided for them (aOR 2.3 p = 0.03, 95% CI 1.04–5.2). Adolescent mothers who had interacted with their health worker were less likely to report PND than those who had no interaction with the health worker (aOR 0.4 P0.02, CI 0.17–0.67). Social and cultural factors can impact mental health of adolescent mothers. It is recommended that targeted and integrated interventions are developed, implemented. and evaluated. There is a need to improve policy and practice to better support adolescent mothers postnatally.