Background Although national data indicate a rising prevalence of type 1 diabetes (T1D), epidemiologic patterns among US Department of War (DoW) beneficiaries remain largely unexamined. The DoW healthcare system provides a unique closed-network environment, enabling comprehensive longitudinal analyses across diverse beneficiary groups, including active-duty personnel, retirees, and dependents. Objective The objective of this retrospective study is to estimate the incidence and prevalence of T1D among DoW beneficiaries from 2018 to 2023, with analyses conducted overall and stratified by age group. Methods T1D cases were identified in the TRICARE dataset by the presence of at least one diagnostic claim coded for T1D (ICD-10-CM: E10.x) between October 2017 and October 2024. Prevalence was assessed for each full calendar year during the study period (2018–2023) and was defined as the number of beneficiaries with T1D per 100,000 persons. Incidence was assessed for calendar years 2019–2023, allowing a 1-year washout period without any T1D diagnosis, and was defined as newly diagnosed cases in each year. To ensure accurate case classification and reduce potential overlap with insulin-treated type 2 diabetes (T2D), individuals with diagnostic codes for T2D (E11, E08–E09, E13) were excluded. Results T1D prevalence among DoW beneficiaries increased from 210 to 249 per 100,000 between 2018 and 2023. Prevalence was notably higher among dependents than active-duty personnel, ranging from 252 to 794 per 100,000 across dependent subgroups, compared with 179 to 274 per 100,000 among active-duty members. Youth demonstrated the highest prevalence, particularly those aged 15–17 years, whose rates increased from 429 to 629 per 100,000 over the study period. Incidence declined modestly from 48 to 36 per 100,000 between 2019 and 2023 but remained highest among youth, especially those aged 10–14 years. Autoantibody (AAb) testing was rare: only 0.10% of DoW enrollees underwent any T1D-related AAb testing between 2017 and 2024, indicating very limited screening activity within the system. More granular incidence and prevalence estimates by year, age group, and beneficiary type, as well as stratified AAb testing rates, will be presented at the conference. Conclusion T1D prevalence has increased among DoW beneficiaries, and although the overall incidence declined, it remained consistently highest among youth. These trends align with national reports indicating increasing T1D prevalence and disproportionately higher incidence in younger individuals compared with adults. The extremely low rate of AAb testing highlights limited early detection and screening activity across the system. Persistent differences across age groups and beneficiary types, particularly higher rates observed among dependents compared with active-duty personnel, underscore the need for strengthened prevention efforts, expanded screening practices, and tailored management strategies to address the growing and unevenly distributed T1D burden within the DoW population.