Objectives: The coronavirus disease 2019 (COVID-19) pandemic has led to a decrease in face-to-face classes worldwide, affecting the mental health of children and their parents. The global pandemic has increased children’s overall use of electronic media. This study an-alyzed the effect of children’s screen time on problematic behaviors during the COVID-19 pandemic. Methods: A total of 186 parents from Suwon, South Korea, were recruited to participate in an online survey. The mean age of the children was 10.14 years old, and 44.1% were females. The questionnaire included questions on children’s screen time, problematic behav-iors, and parental stress. Children’s behavioral problems were evaluated using the Behavior Problem Index, whereas the Parental Stress Scale was used to estimate parental stress. Results: The mean smartphone usage frequency of the children was 5.35 days per week, and the mean smartphone screen time was 3.52 hours per day. Smartphone screen time (Z=4.49, p<0.001) and usage frequency (Z=2.75, p=0.006) were significantly correlated with children’s behavioral problem scores. The indirect effect of parental stress on this relationship was also statistically significant (p=0.049, p=0.045, respectively). Conclusion: This study suggests that children’s smartphone screen time has affected problematic behaviors during the COVID-19 pan-demic. Furthermore, parental stress is related to the relationship between children’s screen time and problematic behaviors.