Background and Objective: According to World Health Organization (WHO), Unhealthy Diet (UD) is a major risk factor for chronic diseases, including cardiovascular disease, cancer, diabetes and other obesity-related conditions. That’s why it’s important to study the UD and the factors associated to it. Our study aim to describe the dietary behavior of young high school students and determine its associated factors. Methods: A cross-sectional study was carried out among students of seven high schools in center-eastern Tunisia during the academic year 2021/2022. The data collection was carried out in April2022 by questionnaires distributed at the end of the course session. The questions concerned the socio-demographic characteristics, food habits and physical activity. The UD was defined by WHO as the consumption of less than five fruits and vegetables per day. Data analysis was done using SPSS software. Results: A total of 740 adolescents were included in this study. The mean age was 17±1.16 years with extremes ranging from 14 to 19 years. A female predominance was noted (60%) with sex-ratio at0.66. Only 37%took breakfast regularly, 19.7% had a full meal at lunch and just 31% had diner. UD was found among 91.6% of the study population. UD was significantly associated with not practicing physical activity (p<10-3), not taking breakfast (p<10-3), not having morning snacks (p<10-3), eating cake (p=0.001), eating sweets (p<10-3), eating fried-food (p=0.001) and not passing exams (p<10-3). At multivariate analysis, eating cake (aOR=4.062; CI [1.685-9.795]; p=0.002), eating fried-food (aOR=3.239; CI[1.118-9.385];p=0.03) and consuming sweets (aOR=3.263; CI[1.666-6.394];p=0.001) were independent risk factors of UD. In the other hand, regular physical activity (aOR=0.524; CI [0.294-0.936]; p=0.029) and having breakfast regularly (aOR=0.523; CI [0.297-0.923]; p=0.025) were independent protective factors of UD. Conclusions: The prevalence of UD is alarming. That’s why it is mandatory to establish effective strategies to support healthy diets and physical activity among adolescents in order to reduce non-communicable diseases related to UD.