A wear indicator (14) comprises an indicating edge (20) which interacts with a reference surface formed by a surface of the pad (7) of the disc brake (1) opposed to the pad (6) with which the indicator is associated. The increase of the wear on the friction material (10, 10') causes said pads (6, 7) to move progressively toward each other. Consequently, the aperture (40) delimited by the indicating edge (20) and by the reference surface (36) is progressively closed. The described wear indicator can be used to measure even small variations of the thickness of the layer of friction material, since, as the wear increases, both the indicating edge (20) and the reference surface (36) move toward each other.