Background: Smartphone ownership had a significant association with problematic use of the internet and internet addiction. Objective: to determine the prevalence of problematic internet use (PIU) through smartphone among adolescents and describe the sociodemographic characteristics affecting it as well as the effect of misuse on family and peer relations. Method: An analytic cross- sectional study was carried out on 454 preparatory and secondary private school students in Sohag City, Egypt. Internet Addiction Test (IAT) was used to measure the internet use. Results: PIU was reported in 52.4% of students whereas internet addiction (IA) occurred in 4% of the students. The mean score of IAT was significantly higher among males than females (54.7±16.9 versus 49.9±15.4 respectively, P= 0.002). Social media activities were significantly higher among males (88.0%) whereas educational internet activities were significantly higher among females (75.2%). PIU and IA through smartphone were significantly higher among adolescents whom fathers were highly educated (56.0% & 5.6% respectively) and among adolescents who had internet access at home (81.9% & 88.9% respectively). Bad/neutral relations with family members were significantly higher among those with PIU and IA (61.3 & 7.5% respectively). Conclusion and recommendations: PIU through smartphones is prevalent among adolescents; males are more involved especially in social media activities. The PIU is significantly higher when internet access is available at home and among those whom fathers are highly educated. Bad family relationship is significantly higher among adolescents with PIU and IA. Interactive and media awareness programs and parents control with restriction of smartphone use among adolescents are required.