The AIM-9 Sidewinder is a short-range air-to-air missile. Entering service with the United States Navy in 1956 and the Air Force in 1964, the AIM-9 is one of the oldest, cheapest, and most successful air-to-air missiles. Its latest variants remain standard equipment in most Western-aligned air forces. The Soviet K-13 (AA-2 "Atoll"), a reverse-engineered copy of the AIM-9B, was also widely adopted.

Low-level development started in the late 1940s, emerging in the early 1950s as a guidance system for the modular Zuni rocket. This modularity allowed for the introduction of newer seekers and rocket motors, including the AIM-9C variant, which used semi-active radar homing and served as the basis of the AGM-122 Sidearm anti-radar missile. Due to the Sidewinder's infrared guidance system, the brevity code "Fox two" is used when firing the AIM-9. Originally a tail-chasing system, early models saw extensive use during the Vietnam War, but had a low success rate (8% hit rate with the AIM-9E variant). This led to all-aspect capability in the L (Lima) version, which proved an effective weapon during the 1982 Falklands War and Operation Mole Cricket 19 in Lebanon. Its adaptability has kept it in service over newer designs like the AIM-95 Agile and SRAAM that were intended to replace it.

The Sidewinder is the most widely used air-to-air missile in the West, with more than 110,000 missiles produced for the U.S. and 27 other nations, of which perhaps one percent have been used in combat. It has been built under license by Sweden and other nations. The AIM-9 has an estimated 270 aircraft kills.

In 2010, Boeing won a contract to support Sidewinder operations through to 2055. In 2021 an Air Force spokesperson said that its relatively low cost, versatility, and reliability mean it is "very possible that the Sidewinder will remain in Air Force inventories through the late 21st century".

The AIM-9 was a product of the US Naval Weapons Center at China Lake in the Mojave Desert. It features a lightweight, compact design with cruciform canards and tail fins. It uses a solid rocket motor for propulsion, similar to most conventional missiles, a continuous-rod fragmentation warhead, and an infrared seeker.

The seeker tracks a difference in temperatures detected and uses proportional guidance to achieve impact. Older variants such as the AIM-9B with uncooled seeker heads could track only the high temperatures of engine exhaust, making them strictly rear-aspect. Later variants, however, featured liquid nitrogen coolant bottles in the launchers, allowing the missile to track any part of the aircraft heated by air resistance due to high-speed flight, giving modern Sidewinders all-aspect capabilities.

The nose canards provide maneuverability for the AIM-9, with the AIM-9X using thrust vectoring to augment this. The hot gases generated were used to actuate the nose canards in older models, while newer variants use thermal batteries.

To minimize the amount of energy devoted to actuating control surfaces, the AIM-9 does not use active roll stabilization. Instead, it uses rollerons, small finned metal discs protruding out of the aft end of the tips of the tail fins which are spun by the passing airflow as the missile flies through the air, providing gyroscopic stabilization.