BACKGROUND: Numerous health apps focusing on fitness, nutrition, and physical activity are available, yet many adolescents have never used them. The purpose of this study was to assess the prevalence of the use of fitness and diet apps and related socio-demographic factors and digital behaviors among high school students. A cross-sectional study. METHODS: High school students were recruited from 4 out of 21 public high schools in Belgrade, Serbia. Participants filled out an anonymous questionnaire about socio-demographic characteristics and digital behaviors, along with the e-health literacy scale (eHEALS). Adjusted logistic regression was applied for data analysis using SPSS 20. RESULTS: Of the 702 students who completed the questionnaire, 670 used smartphones (95.4%; average age 16.5±1.2 years). The prevalence of fitness and diet app use among girls was 29.7% and 9.6%, as well as 17.3% and 3.6% among boys, respectively. Having higher family income, having better e-health literacy, browsing websites about fitness and diet, and using diet apps but not watching YouTube were associated with the use of fitness apps among girls. Being younger and browsing fitness websites and YouTube were associated with the use of fitness apps among boys. Being younger, browsing websites about diet, and using fitness apps were associated with the use of diet apps among girls. CONCLUSION: Based on the findings, girls used fitness and diet apps more often than boys. Practical demonstrations on how to utilize certain health apps could be an additional opportunity to support positive health behaviors among adolescents.