A lock-up clutch is a device which prevents slip in a torque converter, improving efficiency and reducing fuel consumption.

A torque converter is one type of coupling between engine and transmission. It allows slip between the input and output which is desirable when moving away from standstill. But when cruising there is still a small amount of slip in a torque converter, and this wastes fuel. To avoid this, a lock-up clutch can be incorporated to make a mechanical connection between input and output, ensuring that there is no slip. The clutch releases when the gearbox changes down.