Operation Hailstone (Japanese: トラック島空襲, romanized: Torakku-tō Kūshū, lit. 'airstrike on Truk Island'), 17–18 February 1944, was a massive United States Navy air and surface attack on Truk Lagoon conducted as part of the American offensive drive against the Imperial Japanese Navy (IJN) through the Central Pacific Ocean during World War II.

Prior to Operation Hailstone, the IJN had used Truk as an anchorage for its large Combined Fleet. The coral atoll surrounding Truk's islands created a safe harbor where the few points of ingress and egress had been fortified by the Japanese with shore batteries, antiaircraft guns, and airfields.

American estimates of Truk's defenses and its role as a stronghold of the Japanese Navy led newspapers and military men to call it the "Gibraltar of the Pacific", or to compare it with Pearl Harbor. Truk's location in the Caroline Islands also made it an excellent shipping hub for armaments and aircraft moving from Japan's home islands down through the South Seas Mandate and into the Japanese "Southern Resources Area".

By early 1944, Truk was increasingly unsustainable as a forward base of operations for the Japanese. To the west, American and Australian forces under General Douglas MacArthur had moved up through the Southwest Pacific, isolating or overrunning many Japanese strong points as part of Operation Cartwheel. The U.S. Navy, Marine Corps, and Army, under the command of Admiral Chester W. Nimitz, had overrun the most important islands in the nearby Gilbert Islands and Marshall Islands, and then built numerous air bases there.

As a result, the Japanese Navy had to relocate the Combined Fleet's forward base to the Palau Islands, and eventually to Indonesia, and the Fleet had begun clearing its major warships – carriers, battleships, and heavy cruisers – out of Truk before the Hailstone attack struck.

Nevertheless, the Hailstone attack on Truk caught a good number of Japanese auxiliary ships and cargo ships in the harbor, as well as some smaller warships. Between the air attacks and surface ship attacks over the two days of Operation Hailstone, the worst blow against the Japanese was about 250 warplanes destroyed, with the concurrent loss of irreplaceable experienced pilots, and 17,000 tons of stored fuel. Also, about forty ships – two light cruisers, four destroyers, nine auxiliary ships, and about two dozen cargo vessels – were sunk.

Considerable damage was inflicted on the various island bases, including dockyards, communications centers, supply dumps, and its submarine base. Truk remained effectively isolated for the remainder of the war, cut off and surrounded by the American island hopping campaign in the Central Pacific, which also bypassed important Japanese garrisons and airfields in the Bismarck Archipelago, the Caroline Islands, the Marshalls, and the Palaus. Meanwhile, the Americans built new bases from scratch at places like the Admiralty Islands, Majuro, and Ulithi Atoll, and took over the major port at Guam.