The United States Third Fleet is one of the numbered fleets in the United States Navy. Third Fleet's area of responsibility includes approximately fifty million square miles of the eastern and northern Pacific Ocean areas including the Bering Sea, Alaska, the Aleutian Islands, and a sector of the Arctic. Major oil and trade sea lines of communication within this area are critically important to the economic health of the United States and friendly nations throughout the Pacific Rim region.

First established in 1943, the Third Fleet conducted extensive operations against Japanese forces in the Central Pacific during World War II. Deactivated in 1945, the fleet remained inactive until 1973, when it was reactivated and assumed its current responsibilities.

The Third Fleet plans and executes naval operations in the Pacific Ocean. The fleet provides maritime homeland defense, regional security, and humanitarian operations support through integrated naval and coastguard forces acting as a single Sea Service. It strengthens relations between the U.S. and its allies and partners through joint, inter-agency and multinational exercises and operations like Rim of the Pacific, Pacific Partnership, and Fleet Weeks. The Third Fleet ensures realistic, relevant training to its personnel so they have the necessary skills to promote peace and prevail in conflict.

The Third Fleet is a combat-ready power in control of ships, submarines, and aircraft stationed in California, Washington, and Hawaii. The Third Fleet's sea-going force includes five aircraft carrier strike groups, each consisting of a combination of cruisers, destroyers, and frigates. They also have more than 30 submarines and a dozen supply ships to support the strike groups. Third Fleet's air forces comprises more than 400 Navy aircraft, including Boeing F/A-18 Super Hornets, Northrop Grumman E-2C Hawkeyes, McDonnell Douglas AV-8B Harrier II, Bell AH-1Z Viper and Sikorsky SH-60 Seahawk helicopters.

The Third Fleet was originally formed during World War II on 15 March 1943 under the command of Admiral William F. Halsey. Its on-shore headquarters at Pearl Harbor, Hawaii was established on 15 June 1944. The ships of the Third Fleet also formed the basis of the Fifth Fleet, formed on 26 April 1944, which was the designation of the "Big Blue Fleet" when under the command of Admiral Raymond A. Spruance. Spruance and Halsey alternated command of the fleet for major operations, allowing the other admiral and his staff time to plan for subsequent operations. A secondary benefit was confusing the Japanese into thinking that there were actually two separate fleets as the fleet designation flipped back and forth.

While under Halsey's command as the Third Fleet, the fleet operated in and around the Solomon Islands, the Philippines, Formosa, Okinawa, the Ryukyu Islands, and the Japanese Home Islands, first with the battleship USS New Jersey and, from May 1945 to the end of the war, the battleship USS Missouri as its flagship. As the Third Fleet, it took part in the Palau Islands campaign of September–November 1944 and the Philippines campaign of 1944–1945, defeated the Imperial Japanese Navy in two of the four major actions – the Battle of the Sibuyan Sea and the Battle off Cape Engaño – that made up the Battle of Leyte Gulf in October 1944, encountered the damaging Typhoon Cobra in December 1944, endured Typhoon Connie in June 1945, and took part in the war's final operations in Japanese waters in the summer of 1945, launching air attacks on Tokyo, the naval base at Kure, and the island of Hokkaidō and bombarding several Japanese coastal cities with naval gunfire.

The British Pacific Fleet was operating as Task Force 57 of the Fifth Fleet when Halsey relieved Spruance of command in May 1945. Like the rest of the Fifth Fleet's ships, the British ships were resubordinated from Spruance's Fifth Fleet to Halsey's Third Fleet. The British Pacific Fleet then constituted Task Force 37 under the Third Fleet's operational command through the end of World War II on 15 August 1945.

The Third Fleet's next major combat operation would have been Operation Olympic, the invasion of Kyushu in the Japanese Home Islands, scheduled to begin on 1 November 1945, during which it would have operated simultaneously with the Fifth Fleet for the first time.^{[contradictory]} The end of the war made this operation unnecessary.