A fluid coupling is a hydraulic device used for transmitting mechanical shaft power from a rotating driver to a rotating driven load. It has been used in automobile transmissions as an alternative to a mechanical clutch. It also has widespread application in marine and heavy industry. It is generally used in mechanical applications which require start-up or speed changes without resulting in shock loading of the power transmission system.

A fluid coupling is a sealed chamber containing two toroids in a bath of hydraulic fluid. The driving toroid, called (the pump), rotates and the motion of its radial chambers imparts a centripetal motion to the hydraulic fluid. The moving fluid reaches the center of the driven toroid the impeller or turbine, where Coriolis force reaction transfers the angular momentum outward, applying a twisting force (torque) to rotate the pump in the same direction as the turbine. The fluid leaving the outer edges of the pump returns to the turbine, where its cycle begins again.

In an automotive transmission, the turbine is connected to the flywheel of the engine, and is turned by the engine's crankshaft. The pump is connected to the input shaft of the transmission. As engine speed increases, the engine's rotation is transferred to the output shaft by the motion of the fluid.

A fluid coupling is never 100% efficient: some of the motion of the turbine is lost to friction (transformed to heat) within the hydraulic fluid. As a result, the turbine always spins faster than the pump. This speed difference is called slip. Additionally, because the fluid returning to the turbine is moving in the opposite direction of the turbine's rotation, it causes some braking effect, further reducing efficiency.

At very low impeller speeds, such as an engine at idle speed, the slippage of the coupling means that the force imparted to the pump is not sufficient to overcome the rotational inertia of the transmission, and the transmission gears remain stationary. Unlike a mechanical clutch, where the lack of motion in the transmission will stall the engine unless the clutch is disengaged, a fluid coupling cannot stall, allowing the engine to run as if in neutral. At higher speeds, however, the slippage increases the parasitic power losses of the transmission, which is undesirable.

Fluid couplings were used in a variety of early semi-automatic transmissions and automatic transmissions like General Motors' Hydramatic. They have largely been replaced for automotive use by the torque converter, a fluid coupling with an additional component (or components) called a stator that increases the efficiency of the coupling and provides torque multiplication as well as torque transmission.