Background: The remarkable growth of Internet usage in recent years has been accompanied with an increased risk of Internet addiction (IA). IA which is progressively becoming a societal problem affects the more vulnerable group of adolescents and children, resulting in different mental and physical health consequences. Objective(s): The study aimed to assess the prevalence and predictors of IA among high school students in Sohag, Egypt and its effects on their mental health. Methods: A cross-sectional study was conducted among 400 randomly selected Egyptian high school students using a self- administered structured questionnaire that included socio-demographic and Internet use characteristics, in addition to the Arabic versions of Young Internet Addiction Test (IAT) and General Health Questionnaire (GHQ). Results: More than one third (34.8%) of the studied high school students were mild Internet addicts, 21% were moderate Internet addicts and 4% were severe Internet addicts. Having a Facebook account, Internet access on a smart phone, Internet access outside the house, bad family relationship, presence of the Internet at home, being in 2nd grade of high school, and access to the Internet for entertainment were the predictors of IA. A statistically significant positive correlation was found between IA and deterioration of their mental health (r= 0.33, p < 0.001). Conclusions and recommendations: The high prevalence of IA and association with deteriorated mental health shown in the current study warrant special attention and indicate the need for better parenting style and more controlled use of smart phones.