A PT boat (short for patrol torpedo boat) was a motor torpedo boat 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. In the USN they were organized in Motor Torpedo Boat Squadrons (MTBRONs).

The PT boat was very different from the first generation of torpedo boat, which had been developed at the end of the 19th century and featured a displacement hull form. These first generation torpedo boats rode low in the water, displaced up to 300 tons, and had a top speed of 25 to 27 kn (46 to 50 km/h). During World War I Italy, the US, and UK developed the first high-performance gasoline-powered motor torpedo boats (often with top speeds over 40 kn (74 km/h)) and corresponding torpedo tactics, but these projects were all quickly disbanded after the Armistice. World War II PT boats continued to exploit some of the advances in planing hull design borrowed from offshore powerboat racing and by using multiple lightweight but more powerful marinized aircraft-derived V-12 engines were able to grow in both size and speed.

During World War II, PT boats engaged enemy warships, transports, tankers, barges, and sampans. Some were converted into gunboats, which 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".

Primary anti-ship armament on the standard PT boat was four 21-inch Mark 8 torpedoes, each had a 466-pound (211 kg) TNT warhead and had a range of 16,000 yards (15,000 m) at 36 knots (67 km/h). Two twin .50-inch (12.7 mm) M2 Browning heavy machine guns were mounted for anti-aircraft defense and general fire support. Some boats carried a 20 mm (0.79 in) Oerlikon cannon.

Propulsion was via a trio of Packard 4M-2500 and later 5M-2500 supercharged gasoline-fueled, liquid-cooled V-12 marine engines.

Nicknamed "the mosquito fleet" and "devil boats" by the Japanese, the PT boat squadrons were hailed for their daring and earned a durable place in the public imagination that remains strong into the 21st century. Their role was replaced in the U.S. Navy by fast attack craft.