Objective: Children are particularly vulnerable to heat-related illnesses due to their unique physiological and behavioral characteristics. Understanding the epidemiology and clinical features of heat-related illnesses in children is crucial for guiding targeted preventive measures and management strategies. This descriptive study aims to investigate the characteristics of pediatric patients with heat-related illness who were transferred to emergency departments in Japan. Methods: This study was a secondary analysis of the Heatstroke Study, led by the Heatstroke and Hypothermia Surveillance Committee of the Japanese Association for Acute Medicine. This study included pediatric patients (<18 years) with heat-related illness transferred to emergency departments in the summer from 2017 to 2021. We summarized the circumstances of onset, clinical characteristics, and outcomes. Results: Of the 3,154 registered patients, 146 children were included. Of them, 60% were male, with a median age of 15 years (interquartile range, 13–16 years). The most cases occurred in August (47%), and most (80%) were associated with sports activities and with outdoor settings (70%). Cases with a body temperature above 40 °C were rare (3%). Most cases were admitted to hospitals (75% to the general ward and 16% to the intensive care unit), and patients admitted to intensive care unit had altered consciousness with increased serum creatinine. There were two cases of mortality, both of which were out-of-hospital cardiac arrest. Conclusion Most pediatric cases with heat-related illness were middle or high school students, occurred in August, and were related to outdoor sports activity. Patients admitted to hospitals suffered altered consciousness, dehydration, and acute kidney injury.