A sealing unit for a stem of a mono-tube shock absorber includes a first seal having a sealing ring providing an annular sealing lip that projects in a radially and axially inner direction to slidingly cooperate with the stem and ensure a sealing action with respect to pressurized working fluid inside the mono-tube shock absorber; a second seal with a sealing ring having an annular sealing lip, which projects protruding in a radially and axially external direction to slidingly cooperate, with the stem and ensure a sealing action with respect to the outside of the mono-tube shock absorber; a reinforcing ring snapped into an annular seat of the sealing ring. The reinforcing ring is arranged axially locked between the lip and a bottom wall of the second sealing lip; the first and second seals are two physically distinct and separate elements assembled separately inside a body of the shock absorber.