A toothed lock washer, also known as a serrated washer or star washer,^[8] has serrations that extend radially inward or outward to bite into the bearing surface. This type of washer is especially effective as a lock washer when used with a soft substrate, such as aluminium or plastic,^[8] and can resist rotation more than a plain washer on hard surfaces, as the tension between washer and the surface is applied over a much smaller area (the teeth). There are four types: internal, external, combination, and countersunk. The internal style has the serrations along the inner edge of the washer, which makes them more aesthetically pleasing.^[12] The external style has the serrations around the outer edge, which provides better holding power, because of the greater circumference.^[13] The combination style has serrations about both edges, for maximum holding power.^[14] The countersunk style is designed to be used with flat-head screws.^[15]

Tooth lockwashers are also used for ground bonding where a metal lug or object must be electrically bonded to a surface. The teeth of the washer cut through surface oxides, paints or other finishes and provide a gas-tight conductive path. In these applications the washer is not placed under the head of the screw (or under the nut), it is placed between the surfaces to be bonded. In such applications, the tooth washer does not provide any anti-rotation locking features.