Although problematic smartphone use (PSU) is prevalent and associated with mental health and physical activity, there are no studies on its prevalence and associations in Austria. The aim of this study was to evaluate the prevalence of PSU and its associations with mental health in adolescents and young adults. A cross-sectional online survey was performed from 19 April to 27 July 2023, and the data of N = 913 respondents (14.1% male, 82.4% female, 3.5% diverse; median age: 17 [IQR: 15-18]; range: 14-20 years) were included in the analyses. Overall, 38.1% (females: 39.0%, males: 33.3%) of those surveyed were above the cut-off for PSU measured with the Smartphone Addiction Scale (SAS-SV). In addition to screen time, PSU is also associated with depressive symptoms (aOR = 1.46), anxiety symptoms (aOR = 1.86), disordered eating (aOR = 1.55), and alcohol abuse (aOR = 1.71), but not physical inactivity. On the other hand, physical inactivity was associated with depressive symptoms (aOR = 2.48), anxiety symptoms (aOR = 1.74), distress (aOR = 2.02), and low well-being (aOR = 3.25). A total of 37.7% respondents reported being strongly lonely, as measured with the De Jong Gierveld Loneliness Scale. The amount of screen time, but not PSU, was associated with loneliness. In sum, PSU affects more than one-third of adolescents and young adults in Austria and is associated with increased mental health symptoms. However, intensive screen time seems to be more strongly associated with increased mental health symptoms than PSU itself. The study confirms once again that smartphone use is associated with negative effects and that they should be used responsibly.