Increase in time spent on social media and its association with depressive symptoms, especially among young adults, has generated the need to understand how this association occurs in order to support the development of policies to reduce harm and complications. In view of this fact, this study aimed to assess the mediating effect of social media addiction and sleep quality on the association between time spent on social media and depressive symptoms in Brazilian university students. This is a cross-sectional study conducted with 2,823 university students, who provided information regarding time spent on social media, social media addiction, depressive symptoms and sleep quality. The mediation analysis, adjusted for confounding factors, was performed using the PROCESS macro for SPSS to obtain the total (c), direct (c'), and indirect effects (EI1, EI2, and EI3). The results identified an association between time spent on social media and depressive symptoms, mediated by social media addiction (EI1 = 20%) and sleep quality (EI1 = 40%). These findings help expand knowledge about the mechanisms that mutually influence the relationship between time spent on social media and depressive symptoms, supporting the adoption of strategies to reduce harm resulting from excessive social media use.