Sealing device (1) for rolling contact bearings provided with a shield (2) which may be installed onto an outer race (11) of the bearing (12), and presenting a lateral cylindrical wall (21) which is suitable for being arranged in a radially intermediate position between an encoder (16) and a reading sensor (17) of the bearing (12), and a passing window (29) which is obtained through the lateral cylindrical wall (21) in order to permit the sensor (17) to face the encoder (16); a cover (40) made of plastic material being arranged in such a way as to close the window (29), and being selectively mobile in order to effect variations in pressure between the inside and outside of the bearing (12) between two extreme operating positions in at least one of which the cover (40) is suitable for substantially sealing the window (29).