A PT boat (short for Patrol Torpedo boat) was a torpedo-armed fast attack craft used by the United States Navy in World War II. It was small, fast, and inexpensive to build, valued for its maneuverability and speed but hampered at the beginning of the war by ineffective torpedoes, limited armament, and comparatively fragile construction that limited some of the variants to coastal waters.

During World War II, PT boats engaged enemy warships, transports, tankers, barges, and sampans. As gunboats they could be effective against enemy small craft, especially armored barges used by the Japanese for inter-island transport. Several saw service with the Philippine Navy, where they were named "Q-boats" most probably after President Manuel L. Quezon

Primary anti-ship armament was four 2,600 pound (1,179 kg) Mark 8 torpedoes. Launched by 21-inch Mark 18 (530 mm) torpedo tubes, each bore a 466-pound (211 kg) TNT warhead and had a range of 16,000 yards (14,630 m) at 36 knots (66 km/h). Two twin M2 .50 cal (12.7 mm) machine guns were mounted for anti-aircraft defense and general fire support. Some boats shipped a 20 mm Oerlikon cannon.

Propulsion was via a trio of Packard 4M-2500 and later 5M-2500 supercharged gasoline-fueled, liquid-cooled marine engines.

Nicknamed "the mosquito fleet" – and "devil boats" by the Japanese – the PT boat squadrons were heralded for their daring and earned a durable place in the public imagination that remains strong into the 21st century.