BackgroundIt has been documented that social media addiction (SMA) has a detrimental effect on college students’ academic engagement. However, the mechanisms underlying this association are not well understood. This study aimed to determine the serial mediation effects of sleep quality and fatigue on the relationship between SMA and academic engagement among college students.MethodsA cross-sectional survey was conducted with 2661 college students (43.3% males, mean age = 19.97 years). The participants completed the Bergen Social Media Addiction Scale, the Utrecht Student Work Engagement Scale for Students, the Pittsburgh Sleep Quality Index, and the Fatigue Assessment Scale. The serial mediation effects were examined using Model 6 in the Hayes’ PROCESS macro for SPSS.ResultsThe results showed that SMA among college students had a direct negative relationship with their academic engagement (Effect = − 0.051, 95% CI: −0.087 to − 0.015). In addition, sleep quality and fatigue mediated the relationship between SMA and academic engagement both independent and serially, with the independent mediation effect of sleep quality being − 0.031 (95% CI: −0.048 to − 0.016), the independent mediation effect of fatigue being − 0.109 (95% CI: −0.133 to − 0.088), and the serial mediation effect of sleep quality and fatigue being − 0.080 (95% CI: −0.095 to − 0.066). The total indirect effect of the three mediation paths was 80.9%.ConclusionsDecreased academic engagement caused by SMA can be aggravated by poor sleep quality and fatigue. Strengthening supervision and intervention in social media use among college students, supplemented by attention to psychosomatic health, including sleep quality and fatigue could promote their engagement in academic work.