The purpose of this study is to investigate the relationship between nomophobia levels of high school students and their Internet Addiction. This study also investigates the factors including duration of smartphone and mobile Internet use that trigger and create this phenomenon. In order to explore the prevalence of nomophobia among high school students, the survey model was adopted in this study. As part of this survey, 929 high school students were randomly selected among 9th to 12th graders from Turkish high schools in different socioeconomic areas of Afyon, Ankara, and Mardin provinces. In the data collection process, a demographics questionnaire was employed, as well as a nomophobia scale and Internet addiction scale. According to the results, the nomophobia levels of high school students were found to be slightly above average. Pertaining to gender differences, female students have a higher tendency to exhibit nomophobic behaviors compared to male students. Additionally, students' grade levels (which could also be considered as age) has no effect on the prevalence of nomophobia. Considering the duration of smartphone ownership, it was found that the longer the duration of smartphone usage, the higher the risk of exhibiting nomophobic behaviors. Finally, the results indicated that the higher the level of student Internet addiction, the more nomophobic behaviors they tend to exhibit.