Introduction. Although the COVID-19 pandemic negatively impacted the mental health of vulnerable populations, such as adolescent mothers, very few studies have documented the prevalence of postpartum depression (PPD) in this population. Objective. a) Determine the frequency of PPD (Edinburgh Postnatal Depression Scale [EPDS] ≥ 9) in adolescent mothers before (AM-BP) and during (AM-DP) the pandemic, b) Examine psychosocial factors (self-esteem, maternal efficacy, social support, depression and anxiety in pregnancy, planned and wanted pregnancy) in AM-BP and AM-DP, and c) Determine whether being an AM-DP was a significant factor for experiencing PPD (EPDS ≥ 9). Method. Cross sectional study. Subjects: Forty-one AM-BP recruited at Health Centers and interviewed face to face and forty-one AM-DP surveyed online. Results. PPD (EPDS ≥ 9) was 42% (p = .001) more frequent in AM-DP. The groups differed significantly in all psychosocial factors, with AM-DP faring worse. Unadjusted regressions showed that being an AM-DP, having lower maternal efficacy and self-esteem, greater dissatisfaction with social support, and depression and/or anxiety in pregnancy increased PPD (EPDS ≥ 9). Adjusted multiple analysis indicated that lower self-esteem was the only factor to maintain its association with PPD (EPDS ≥ 9; p = .017). Discussion and conclusion. The pandemic negatively affected PPD (EPDS ≥ 9) and psychosocial factors in AM-DP, as compared to AM-BP, with self-esteem being the main factor associated with PPD (EPDS ≥ 9). In situations of extreme stress as happened in the pandemic, the mental health of adolescent mothers should be prioritized to prevent negative effects such as PPD. PPD preventive and treatment interventions should consider strengthening self-esteem.