BACKGROUND: Tuberculosis elimination strategies in Brazil might neglect adolescents and young adults aged 10-24 years, hampering tuberculosis control. However, little is known about factors associated with tuberculosis treatment outcomes in this underserved group. In this study, we aimed to investigate social and health factors associated with unfavourable treatment outcomes in young people with tuberculosis in Brazil. METHODS: A national retrospective cohort study was done using data from Sistema de Informação de Agravos de Notificação (SINAN), the national tuberculosis registry in Brazil. People aged 10-24 years (young people) with tuberculosis registered in SINAN between Jan 1, 2015, and Dec 31, 2018, were included. Unfavourable outcomes were defined as loss to follow-up, treatment failure, and death. Favourable outcome was defined as treatment success. Multiple logistic regression models estimated the association between social and health factors and tuberculosis treatment outcomes. FINDINGS: 67 360 young people with tuberculosis were notified to SINAN, and we included 41 870 young people in our study. 7024 (17%) of the 41 870 included individuals had unfavourable treatment outcomes. Young people who received government cash transfers were less likely to have an unfavourable outcome (adjusted odds ratio 0·83, 95% CI 0·70-0·99). Homelessness (3·03, 2·07-4·42), HIV (2·89, 2·45-3·40), and illicit drug use (2·22, 1·93-2·55) were the main factors associated with unfavourable treatment outcome. INTERPRETATION: In this national cohort of young people with tuberculosis in Brazil, tuberculosis treatment success rates were lower than WHO End TB Strategy targets, with almost a fifth of participants experiencing unfavourable treatment outcomes. Homelessness, HIV, and illicit drug use were the main factors associated with unfavourable outcome. In Brazil, strategies are required to support this underserved group to ensure favourable tuberculosis treatment outcomes. FUNDING: Wellcome Trust, UK Medical Research Council, and UK Foreign Commonwealth and Development Office.