A growing body of literature has demonstrated relations between mood- and anxiety-related psychopathology with problematic smartphone use (PSU) symptom severity. However, there has been little empirical inquiry of potential mediators of these relationships. The current study examined trait mindfulness and smartphone use expectancies as mediators of the relation between depression/anxiety and PSU severity in 352 undergraduate students. Participants completed an online survey that measured depression, anxiety, smartphone use expectancies, and PSU severity. Structural equation modeling demonstrated that trait mindfulness was inversely associated, and smartphone use expectancies were positively associated, with PSU severity. Trait mindfulness significantly mediated relations between anxiety and PSU severity. Results provide implications for understanding PSU within the context of theoretical models of PSU’s development, and highlight the role of mindfulness as an emotion regulation strategy and potential treatment for PSU