The Baltic Security Initiative Act establishes an initiative under the Secretary of Defense to deepen security cooperation with the military forces of Estonia, Latvia, and Lithuania. Its primary objectives are to deter aggression by the Russian Federation and implement the North Atlantic Treaty Organization's new Strategic Concept, thereby strengthening the alliance's deterrence and defense posture. This initiative seeks to deny potential adversaries any possible opportunities for aggression and enhance regional planning and cooperation among the Baltic countries' military forces. Key provisions include enhancing regional cooperation on long-term capability projects such as long-range precision fire systems, integrated air and missile defense, and improving cyber defenses and resilience to hybrid threats. The Secretary of Defense must submit a strategy report within one year, considering ongoing security threats from Russia and the People's Republic of China. The bill authorizes $350,000,000 annually for fiscal years 2026 through 2028, with a Sense of Congress encouraging matching funds from participating Baltic countries.