Background: Adolescence refers to the developmental period between childhood and adulthood, and the WHO defines adolescents as individuals between the ages of 10–19 years. They make up about 20% of the world population. Rapid development and urbanization of KSA has resulted in changes in nutritional and diet patterns that have affected the BMI of children and adolescents and led to nutritional transition. The objective of this study was to evaluate and estimate the weight status and prevalence of underweight in school-aged male and female adolescents in Arar city, Northern Saudi Arabia using the BMI. Methods: This was a cross-sectional study that has been conducted among school adolescents of both sexes, aged 12-18 years during the academic year 2015–2016 over a period of 9 months (October 1st, 2015 to June 30th, 2016). Information obtained through an interviewer administered semi-structured questionnaire. Body mass index (BMI) was calculated as per the formula BMI = weight (kg) / height(m2). Age and sex-specific BMI percentiles were computed based on the Centre for Disease Control (CDC)/National Centre for Health Statistics growth curves. Results: In the studied participants (n = 849), the male-female ratio was (48.1%:51.9%). About half (50.4%) of the studied adolescents had normal BMI, 25.7% were underweight (below the 5th percentile), 18.1% were overweight and only 5.8% were obese (above 95th percentile). In conclusion; nutritional status among adolescents is not so well. Yet more work is needed to identify the more influential factors which can improve the nutritional status among adolescents in Arar beside nutritional education programs to the mothers and/or care givers.