During the COVID-19 pandemic, nationally collected data revealed disproportionate rates of COVID-19 testing and infection by race, socioeconomic status, and sex. Despite the abundance of evidence for COVID-19 disparities within the U.S. civilian population, there is limited evidence to determine whether these disparities were also present within the Military Health System (MHS). This study investigated disparities in COVID-19 testing and infection rates within the universally insured, MHS population during the first year of the pandemic. We performed a retrospective cohort study of TRICARE beneficiaries, aged 18-64 years, with a medical encounter between March 1, 2020 and February 28, 2021. Data from the MHS Data Repository (MDR) were used to identify all PCR-based COVID-19 tests administered within the direct care sector during the specified period. We employed logistic regression modeling to examine the likelihood of COVID-19 testing and infection by race, sponsor rank (as a proxy for socioeconomic status), and sex with adjustments for age and beneficiary status. During the study period, 697,899 individuals were tested for COVID-19 within direct care, of whom 56,047 (8%) tested positive. Among those tested, 70% were male, 61% were White, and 83% were Junior or Senior Enlisted rank. Females were more likely to be tested than males (OR: 1.23; 95 % CI: 1.21-1.24). Individuals from minority backgrounds were more likely to be tested than White individuals (Black, OR: 1.07; 95% CI: 1.07-1.08; Asian/Pacific Islander, OR: 1.23; 95% 1.21-1.24). Relative to Senior Enlisted, Junior Enlisted were less likely, while Junior Officers were more likely to be tested (OR: 0.73; 95% CI: 0.73-0.74 and OR: 1.20; 95% CI: 1.18-1.21, respectively). Females were less likely than males to test positive (OR: 0.87, 95% CI: 0.85-0.89). After multivariable adjustment, Black individuals were more likely to test positive, while Asian/Pacific Islanders were less likely to test positive than Whites (Black, OR: 1.10, 95% CI: 1.07-1.13; Asian/Pacific Islander, OR: 0.94; 95% CI: 0.91-0.98). Officer ranks were less likely than Senior Enlisted to test positive for COVID-19 (Junior Officer, OR: 0.92, 95% CI: 0.89-0.95; Senior Officer, OR: 0.70, 95% CI: 0.67-0.74). Our results indicate that disparities in COVID-19 testing and infections persisted within the MHS, despite universal insurance and healthcare access. Given that our findings mostly aligned with studies on COVID-19 disparities within the civilian population, it is possible that there are similar factors underlying the observed inequities. Further research into the factors that may contribute to lower testing utilization and disproportionate rates of infection within the MHS, such as occupational demands, perceptions of the healthcare system, and individual health behaviors is imperative. Future outbreaks in the MHS may be better managed by proactively developing targeted outreach programs aimed at preventing disparities in testing utilization and infection rates among vulnerable groups. Finally, implementing policies and practices within the MHS that promote equitable distribution of testing and preventive measures for all beneficiaries may lead to improved outcomes. Disclaimer: The contents of this publication are the sole responsibility of the authors and do not necessarily reflect the views, assertions, opinions or policies of the Uniformed Services University of the Health Sciences (USUHS), the Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc. (HJF), the Department of Defense (DoD), or the Departments of the Army, Navy, or Air Force. Mention of trade names, commercial products, or organizations does not imply endorsement by the U.S. Government.