Background and aims: Due to the rapid spread of the novel coronavirus disease 2019 (COVID-19), policies based on the nature of "spatial distancing" have been implemented and have resulted in school suspensions and online learning among schoolchildren. In order to examine the impact of such policies on schoolchildren, the aims of the present study were to (i) assess changes in the level of engagement in three internet-related activities (smartphone use, social media use, and gaming) before and during the COVID-19 outbreak, including prolonged and problematic engagement in these activities; (ii) investigate the differences of psychological distress before and after COVID-19 outbreak; and (iii) to use structural equation modeling to investigate the mediating roles of problematic internet-related behaviors in the causal relationships of psychological distress and time spent on internet-related activities. Methods: Self-report measures were used to assess internet-related activities and psychological distress. Time spent on internet-related activities, problematic use of internet-related activities, and psychological distress were collected from primary school students (N 5 535; 265 boys; M age 5 10.32 years [SD 5 0.84]). The data were first collected before the COVID-19 outbreak (i.e., early November 2019) and then collected again during the school suspension due to COVID-19 outbreak (i.e., end of March 2020) for comparisons of changes. Results: Schoolchildren spent significantly more time on the smartphone (increased 1.02 h daily; P < 0.001) and social media (increased 0.73 h daily; P < 0.001) but not gaming (increased 0.14 h daily; P 5 0.07) during the school suspension compared to the baseline. Schoolchildren who increased by 15 or 30 min daily on internet-related activities showed an increased level of psychological distress. The association between problematic use of social media and psychological distress was stronger during the school suspension (b 5 0.584) than at the baseline (b 5 0.451; P < 0.001). Conclusion: Increased problematic use of internet-related activities among schoolchildren was associated with greater psychological distress. Parents should therefore monitor internet-related activities and psychological distress of their children to support their mental health. Kulcsszavak: child, COVID-19, problematic internet use, psychological distress, school suspension