Smartphones are omnipresent and serve as a tool for various purposes. In Switzerland, more than 90% of the population owns a smartphone and uses it on average around 2.0 to 5 h daily. Smartphone app use is linked with both higher and lower levels of well-being. However, it remains unclear whether smartphone app use is the antecedent or the consequence of well-being. Based on the stimulation (online communication increases well-being) and the displacement (online communication decreases well-being) hypotheses, this study explores how daily smartphone app use and well-being are associated over time. A total of N = 130 employees (M = 35.61; SD = 10.62) participated in a three-week intensive longitudinal study. Daily minutes of different types of smartphone app use (e.g. email apps and chat apps) were tracked by a mobile application. Furthermore, daily self-reports of positive and negative affect were assessed in a morning and end-of day diary. Positive morning affect was negatively associated with the use of email apps. Furthermore, a negative between-person relationship of chat app use with end-of day positive affect was observed. The results are in favour of the displacement hypothesis. Future studies should investigate causal relationships between specific forms of smartphone usage and the user’s well-being.