Background: Even though a number of studies have verified the positive effect of physical activity (PA) on self-related health (SRH) no previous research has examined this association among pediatric patients with Type 1 diabetes mellitus (T1DM). Objective: The purpose of this study was to investigate the association between regular physical activity (PA) and self-rated health (SRH) in pediatric patients with Type 1 diabetes mellitus (T1DM) who lacked diabetes care. Methods: We conducted a retrospective study among pediatric patients with T1DM who lacked diabetes care and were enrolled in a diabetes education program between January 2011 to January 2015 at the endocrinology clinic of Seoul National University Children’s Hospital in South Korea. The eligible participants for this study were 37 pediatric patients with T1DM aged 9 to 17 years. PA was divided into regular PA and muscle strength exercise to analyze the relationship with SRH using binomial logistic regression analysis. Results: The results showed SRH of pediatric patients with T1DM who did not engage in regular PA was significantly lower than those who did (OR in regular PA = .199 [95% CI: .040, .995]; OR in regular muscle strength exercise = .097 [95% CI: .023, .825]). Conclusions: In conclusion, regular PA and muscle strength exercise in pediatric patients with T1DM who lacked diabetes care were effective in improving their SRH. A systematic plan is required to enhance regular PA for pediatric patients with T1DM.