In South Africa, high rates of adolescent pregnancy and HIV pose prominent public health challenges with potential implications for mental wellbeing. It is important to understand risk factors for mental health difficulties among adolescent mothers affected by HIV. This study aims to identify the prevalence of likely common mental disorder among adolescent mothers (both living with and not living with HIV) and explores hypothesised risk factors for likely common mental disorder. Cross-sectional data from adolescent mothers (10-19 years; n=1002) utilised within these analyses are drawn from a cohort of young mothers residing in the Eastern Cape Province, South Africa. All mothers completed a detailed questionnaire consisting of standardised measures of sociodemographic characteristics, mental health, and hypothesised risk factors. Logistic regression models were utilised to explore associations between hypothesised risk factors and likely common mental disorder. Risk factors were clustered within a hypothesised socioecological framework and entered into models using a stepwise sequential approach. Interaction effects with maternal HIV status were additionally explored. The prevalence of likely common mental disorder among adolescent mothers was 12.6%. Adolescent mothers living with HIV were more likely to report likely common mental disorder compared to adolescent mothers not living with HIV (16.2% vs 11.2%, X2=4.41, p=0.04). Factors associated with likely common mental disorder were any abuse exposure (OR=2.54 [95%CI:1.20-5.40], p=0.01), a lack of perceived social support (OR=4.09 [95%CI:2.48-6.74], p=<0.0001), and community violence exposure (OR=2.09 [95%CI:1.33-3.27], p=0.001). There was limited evidence of interaction effects between risk factors, and maternal HIV status. Violence exposure and a lack of perceived support are major risk factors for poor mental health among adolescent mothers in South Africa. Violence prevention interventions and social support may help to reduce risk. Identified risk factors spanning individual, interpersonal, and community levels have the potential to impact adolescent maternal mental health.