Objectives: In this study, we sought to determine the prevalence of problematic Internet use (PIU) among high school students and identify demographic and socioeconomic factors related to PIU. Methods: Using a cross-sectional design, we conducted this study between November 2017 and January 2018 of 1412 students attending high schools in the Silivri District of Istanbul in Turkey. We administered a questionnaire inquiring about sociodemographic information, Internet use, and Young's Internet Addiction Test to participating students. Results: Among participants, 18.5% (male = 17.2%; females = 19.8%) were found to show PIU. PIU rates were higher among those with a high household income and lower among those who studied in science high schools, performed physical activities at least 2 days a week, and read at least one book a month. PIU rates were also higher among those who had their own bedrooms, phones, smartphones, Internet connections at home, and social networking accounts. Conclusion: The prevalence of PIU among the participants was 18.5%. Female sex, high household income, low physical activity, grade level, school type, reading fewer than one book per month, and Internet use experience were risk factors for PIU. PIU is considered a significant public health concern across the world, including in Turkey.