Phubbing is the act of momentarily engaging with the smartphone during face-to-face conversations. Past research on phubbing treated this behaviour as a stable disposition, rather than a temporary impulse and because of that, the predictors investigated to explain this behaviour were also trait predictors. The present study treated phubbing as a temporary reaction. Furthermore, instead of examining stable dispositions to explain this behaviour, the study investigated the effect of the fleeting feeling of boredom and state fear of missing out (FoMo) on this momentary behaviour, which past research found they correlated. The purpose of this study was to investigate the mediating role of state FoMo in the relationship between state boredom and state phubbing. Data were collected from 325 smartphone users who completed an online survey. In addition to completing the state phubbing scale, the participants also completed the shortened version of the state boredom and the state of fear of missing out scales. The mediation analysis revealed that state FoMo partially mediated the effect of state boredom on state phubbing. That is, part of the effect of state boredom on state phubbing was transmitted to state phubbing through state FoMo. In serving as a mediator in the relationship between state boredom and state phubbing, state FoMo can be viewed as the excuse to phub in order to avoid or alleviate the negative feeling of state boredom. Future research can build on this finding to experimentally investigate the role of state FoMo in the relationship between state boredom and state phubbing.