Background: Diabetes mellitus (DM) is rapidly becoming one of the main health issues among humans in the 21^st  century. The  increase in the incidence of type 1 diabetes has been observed as a global public health problem in children and adolescents. This work was conducted aimed to estimate the prevalence of type I diabetes and to describe some related characteristics of cases in a sample of adolescent primary and secondary school girls of Riyadh city, KSA. Methods: A cross-sectional study was conducted during the academic year 2016-2017. Data were collected via a predesigned and pretested questionnaire to gather the relevant data.  The total number of participants was 154 adolescent girls. Results: The overall prevalence of  type I diabetes among the studied adolescent girls was 5.2% with Mean(±SD) age was 14.08 (± 3.4). All diabetic girls were Saudi. Only 25% of the cases were using a hormonal contraception. No smoking history or other chronic diseases was detected among them. Conclusion: The present study establishes the prevalence of type 1 DM among Saudi adolescent girls to be 5.2%. We suggest repeating this study periodically, with concentration on the various possible etiological and risk factors. Also we recommend conducting similar studies in other areas of Saudi Arabia to get more information about Diabetes. Awareness campaigns and continuous medical education is of utmost importance to detect the disease to guard against development of complications.