A timing belt is a part of an internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft so that the engine's valves open and close at the proper times during each cylinder's intake and exhaust strokes.

In an interference engine the timing belt or chain is also critical to preventing the piston from striking the valves. A timing belt is usually a toothed belt—a drive belt with teeth on the inside surface.

The use of a timing belt or chain instead of a gear drive enables engine designers to place the camshaft further from the crankshaft, and in engines, with multiple camshafts a timing belt or chain also enables the camshafts to be placed further from each other.