Adenovirus (AdV) outbreaks have long been a cause of acute respiratory disease, hospitalization and death in otherwise young, healthy military recruits. The administration of oral, live attenuated AdV vaccine against AdV types 4 and 7 has been critical in preventing outbreaks in this population. In early July 2024, a spike in recruit hospitalizations from the Marine Corps Recruit Depot San Diego (MCRD San Diego) for AdV pneumonia were recognized and an outbreak investigation commenced. From July 1 through September 23, 2024, a total of 212 AdV cases including 28 hospitalizations, were identified among trainees and staff of MCRD San Diego. Non-pharmaceutical interventions, including aggressive environmental cleaning, separation of sick and well recruits, and masking were implemented. However, the outbreak was not appreciably slowed until shifting the AdV vaccine administration from day 11 to day 1 post-arrival of recruits to MCRD San Diego. This outbreak demonstrated that early AdV vaccination for newly arriving recruits is an effective and essential step to prevent AdV morbidity and mortality in the recruit training setting.