An automatic transmission (often informally shortened to auto, and abbreviated to AT) is a motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment.

Most automatic transmissions have a defined set of gear ranges, often with a parking pawl feature that locks the output shaft of the transmission. Continuously variable transmissions (CVTs), which are very different to conventional automatic transmissions, can change their 'ratios' over a wider 'stepless' range, rather than between a set of fixed gear ratios. CVTs have been used for decades in two-wheeled scooters, but have only seen use in a few automobile models. Recently, however, CVT technology has gained greater acceptance among manufacturers and customers, especially in Audi and Nissan automobiles, and gas-electric hybrid vehicles.

Some machines with limited speed ranges or fixed engine speeds, such as some forklift trucks and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.