Introduction: Recently studies examined mediating psychological constructs accounting for relations between both depression and anxiety with problematic smartphone use (PSU) severity. The aim of the current study was to analyze the fear of missing out (FOMO) as a possible mediator in these relationships. Method: We recruited 1034 Chinese undergraduate students via a web-based survey that measured smartphone use frequency, PSU, depression, anxiety and FOMO. Results: Structural equation modeling demonstrated that FOMO was significantly related to smartphone use frequency and PSU severity. FOMO significantly mediated relations between anxiety and both smartphone use frequency and PSU severity. FOMO did not account for relations between depression and smartphone use/PSU. Conclusion: This is one of the first studies testing FOMO in relation to PSU severity among Asian participants. FOMO may be an important variable accounting for why some types of psychopathology (e.g., anxiety) are associated with PSU.