The Japan Maritime Self-Defense Force (JMSDF) is an institution, specifically the navy branch of the Japan Self-Defense Forces (JSDF), tasked with the naval defense of Japan. Its origin is rooted in Japan's post-World War II pacifist constitution. The JMSDF was formally created on July 1, 1954, following the passage of the 1954 Self-Defense Forces Law. This formation occurred nine years after the dissolution of the Imperial Japanese Navy in 1945. The creation solved the problem of providing for national defense under Cold War pressure while navigating the constraints of Article 9 of the 1947 Constitution, which renounces war as a sovereign right and the threat or use of force to settle international disputes.
The JMSDF operates under the legal constraint of Article 9, limiting its functions primarily to self-defense, disaster response, and peacekeeping. A key mechanism change occurred with the 2015 Legislation for Peace and Security, which amended the law to permit the exercise of the right to collective self-defense to assist an allied country in an "existential crisis situation". The JMSDF is fundamentally connected to the larger Japan Self-Defense Forces and maintains a close alliance with the United States.
The institution has undergone significant recent change. On March 23, 2026, the JMSDF completed its "largest-ever reorganization" since 1954. This reform disbanded the long-standing Fleet Escort Force (established in 1961) and the Mine Warfare Force, consolidating them into a new Fleet Surface Force. This shift aims for a more integrated, combat-ready fleet. Furthermore, in December 2018, Japan announced the refitting of its Izumo-class destroyers to carry F-35B fighter jets, effectively making them aircraft carriers. Japan is also acquiring Tomahawk cruise missiles to enhance its long-range stand-off capability.