Internet gaming disorder (IGD) and problematic smartphone use (PSU) symptoms are associated with increased depression symptom severity across studies. The current study aimed to examine the fear of missing out (FoMO) as a possible mediator in this relationship. We employed a three-wave longitudinal design among 341 Chinese university students. Results indicated that depression severity was significantly related to increased symptoms of PSU and IGD symptoms. FoMO significantly mediated the linkage between depression and PSU severity. IGD symptoms also partially mediated the relation between FoMO and PSU severity. This longitudinal study demonstrates the key role of FoMO in the linkage between psychopathology symptoms and Internet-related overuse.