Automatic transmission fluid (ATF), also known as transmission fluid, is the fluid used in vehicles with self-shifting or automatic transmissions. It is typically coloured red or green to distinguish it from motor oil and other fluids in the vehicle.

The fluid is optimized[1] for the special requirements of a transmission, such as valve operation, brake band friction, and the torque converter, as well as gear lubrication.

ATF is also used as a hydraulic fluid in some power-assisted steering systems, as a lubricant in some 4WD transfer cases, and in some modern manual transmissions.

Modern ATF consists of a base oil plus an additive package containing a wide variety of chemical compounds intended to provide the required properties of a particular ATF specification. Most ATFs contain some combination of additives that improve lubricating qualities, such as anti-wear additives, rust and corrosion inhibitors, detergents, dispersants and surfactants (which protect and clean metal surfaces); kinematic viscosity and viscosity index improvers and modifiers, seal swell additives and agents (which extend the rotational speed range and temperature range of the additives' application); anti-foam additives and anti-oxidation compounds to inhibit oxidation and "boil-off"[5] (which extends the life of the additives' application); cold-flow improvers, high-temperature thickeners, gasket conditioners, pour point depressant and petroleum dye.