The U.S. Public Health Service (USPHS) Commissioned Corps played a critical role in advancing health security, disaster preparedness, and global health diplomacy during the U.S. Navy’s Pacific Partnership 2025 (PP25) mission in Tonga and Samoa. From June 18–27, 2025, Public Health Service (PHS) officers specializing in dietetics, pharmacy, environmental health, and engineering joined counterparts from the U.S. Navy, Army, and Air Force to collaborate with Tonga’s Ministry of Health and Vaiola Hospital in Nuku‘alofa. From September 20-November 1, 2025, a PHS engineering officer mobilized to Samoa to support infrastructure and readiness objectives. The mission sought to enhance Indo-Pacific preparedness and resilience through joint health security cooperation, capacity building and force readiness. PHS officers achieved these goals through targeted, evidence-based interventions that delivered measurable improvements in clinical operations, environmental safety, and infrastructure sustainability. The PHS dietitian officer collaborated with a U.S. Navy dietitian officer to lead a hospital wide-wide review of nutrition services, aligning foodservice operations with World Health Organization and national guidelines training 25 staff members in cost-effective menu planning, and reducing portion sizes by approximately 20%, yielding an estimated annual cost savings of 30,000 Tongan pa‘anga. In close coordination with Navy pharmacists, USPHS pharmacy officers standardized 17 medication compounding forms, created a transfusion-reaction management guide, implemented a digital inventory and consumption tracking system that reduced expired drug loss by 22% and trained 25 staff on hazardous drug handling and spill management enhancing system efficiency and patient protection. The PHS environmental health officer led efforts to improve water safety and vector control by restoring emergency treatment capacity for 1 million liters of safe drinking water, rehabilitating groundwater protection barriers, and conducting vector surveillance across the island, thereby by strengthening Tonga’s disease-prevention infrastructure and outbreak preparedness. In Samoa, the PHS engineering officer worked closely with Seabees in design and installation of an underground septic tank and soakage pit system for the Magiagi Primary School, overcoming resource and spatial constraints to provide a durable, low-maintenance sanitation solution that improved hygiene, reduced contamination risks and enhanced the daily health conditions within the Samoan community. Collectively, these outcomes highlight how PHS officers uniquely enhance interservice operations through clinical excellence, engineering innovation, and environmental protection. The success of Pacific Partnership 2025 demonstrates the USPHS Commissioned Corps’ operational readiness and its distinct capacity to strengthen health systems, promote sustainable infrastructure, and advance global health security objectives through culturally competent, outcome-driven collaborations across the Pacific region.