The present study investigates how excessive smartphone use predicts users' stress and loneliness while taking the moderating role of online self-disclosure into account. We conducted a two-wave panel survey with a quota sample of smartphone users (n = 461 at time 2 [T2]). We found no direct effects of excessive smartphone use on stress or loneliness. However, our results showed that online self-disclosure moderates the postulated relationships. That is, excessive smartphone use (time 1 [T1]) predicts increased stress (T2), for those smartphone users who do engage in little online self-disclosure. However, for those who communicate their feelings, anxieties, and problems online, excessive smartphone use (T1) can even reduce loneliness (T2) and relieve stress (T2).