A sealing device is to be fitted at a side of a bearing unit in order to seal a gap (15) between a stationary race (11) and a rotatable race (10) of the bearing unit. The device includes a relatively rigid reinforcing insert (16) of disc-annular shape, and a gasket (17) of relatively soft rubber or elastomeric material, over-moulded or vulcanized onto the insert (16). The gasket forms two opposite peripheral sealing portions (18, 19) intended to perform a static sealing action against the rotatable race (10) and a dynamic sealing action on the stationary race (11), respectively. The insert (16) includes magnetized or magnetizable material for forming, at predetermined angular zones or fields, a sequence of alternate and/or spaced north/south poles, in order to act as an encoder.