It has been shown that both fear of missing out (FoMO) and problematic (i.e., excessive) smartphone use (PSU) are negatively associated with indicators of emotional well-being. Moreover, FoMO has been found to be a key predictor of PSU. This suggests that PSU may mediate the relation between FoMO and decreased emotional well-being but this pathway has never been tested. Moreover, in most studies on PSU, the multidimensional nature of this construct has been ignored. The aim of the present study was to address these gaps by directly testing the mediating role of (subdimensions of) PSU in the association between FoMO and emotional well-being. We conducted a cross-sectional study with Estonian participants (n = 426). Using a simple mediation analysis, we found that PSU partially mediated the relationship between FoMO and decreased emotional well-being. Using a parallel mediation analysis, we found that two specific dimensions of PSU were significant mediators of the relationship between FoMO and decreased emotional well-being: Cyberspace-oriented Relations and Physical Symptoms. This suggests that the negative relationship between FoMO and decreased emotional well-being is due to FoMO stimulating (a) online relationships at the cost of offline interactions and (b) Physical symptoms associated with excessive smartphone use. Overall, this study provides a fine-grained analysis of the relationship between FoMO, PSU and emotional well-being.