Background: stress hyperglycemia (SH) is a relatively frequent finding in pediatric patients. The purpose of this prospective observational study was to identify the prevalence of pediatric SH and its associated risk factors in Greece. Methods: A total of 1005 patients without diabetes who were admitted consecutively for acute illness in a Pediatric Emergency Department were included in the study. Medical history, anthropometric measurements, blood glucose levels, and the medication administered were recorded. A questionnaire was distributed to parents regarding medical and perinatal history and sociodemographic characteristics. Results: There were 72 cases of SH on admission (7.2%) and 39 (3.9%) during hospitalization. Mean age was 6.4 years; 50.3% were male. SH on admission was associated with oral corticosteroid therapy (21.1% vs. 4.7%, p < 0.001), inhaled corticosteroids (12.7% vs. 3%, p < 0.001), and inhaled β2-agonists (30.6% vs. 10.7%, p < 0.001). In-hospital hyperglycemia was associated with oral corticosteroids (adjusted OR = 3.32), inhaled corticosteroids (OR = 10.03) and inhaled β2-agonists (OR = 5.01). Children with asthma were 5.58 and 7.86 times more likely to present admission and in-hospital hyperglycemia, respectively. Conclusions: This is the first report of SH prevalence in pediatric patients in Greece. Asthma, corticosteroids, and β2-agonists significantly increase the risk of SH. No parental factors seem to predispose to SH.