A sealing device for a hub/wheel assembly with a roller bearing. The sealing device includes a guard, made of plastic. The guard is coupled to an outer ring of the bearing to seal off the bearing. The guard has a cylindrical mounting wall with an outer lateral surface coupled to an inner lateral surface of a collar of the outer ring, and a snap-engagement step for engaging with the collar made along the outer lateral surface so that it snap-fits in a contoured annular groove made along the collar about the axis. An annular gasket is made of elastomer and co-injection-molded inside an annular groove made in the cylindrical mounting wall on the same side as the snap-engagement step.