Introduction: A booster dose of messenger RNA vaccine protects against severe COVID-19 outcomes. This study examined the incidence of COVID-19 booster vaccination among active-duty U.S. military servicemembers between August 2021 and January 2022, factors associated with vaccination uptake, and trends over time. Methods: This was a retrospective cohort study of active-duty military personnel using data from the Defense Medical Surveillance System. Participants were included if they served in the active component from August 2021 through January 2022 and were eligible to receive a COVID-19 booster dose by January 2022. Adjusted hazard ratio estimates of time to booster vaccination were calculated using Cox proportional hazards regression. Results: Lower booster vaccine uptake was seen in the U.S. military (25%) than among the general U.S. population at the same time (45%). Booster vaccination increased with older age, with greater education, with higher income, among women, and among those stationed overseas; it decreased with previous COVID-19 infection and use of the Janssen vaccine. There were no significant racial or ethnic disparities in booster vaccination. Conclusions: In the absence of a compulsory vaccination policy, lower booster vaccine uptake was seen among servicemembers than among the general U.S. population, particularly among members who were younger, were male, Marines, and had a previous history of infection. Low vaccination rates not only increase the risk of acute and long-term health effects from COVID-19 among servicemembers, but they also degrade the overall readiness of the U.S. military.