A shock absorber comprising: a tubular base; a piston assembly comprising a piston mounted within the tubular base with an alternate sliding motion, said piston defining within the tubular base a working chamber and an accumulation chamber arranged on the rear side and on the front side of the piston respectively, and a stem connected to the piston; a plurality of fluid paths for connecting the working chamber with the accumulation chamber, at least one of which establishes a permanent fluid communication between the working chamber and the accumulation chamber; and valve means arranged on one of said fluid paths, hereinafter valved fluid path, and comprising a ring-shaped obstructing member arranged around the piston and capable of sliding along an axial length of the piston, wherein said valved fluid path comprises a path portion formed between the piston and the obstructing member, said valved fluid path being thereby capable of selectively assuming an open configuration, in which said valved fluid path establishes a fluid communication between the working chamber and the accumulation chamber, and a closed configuration in which said valved fluid path is blocked; wherein said obstructing member comprises a bush of plastic material, which has a flexible lip portion extending from a rear end of the bush and having a radially outer surface tapered towards said rear end of the bush; said shock absorber being characterized by further comprising a support ring member arranged coaxially with the bush and movable with respect thereto, said support ring member comprising a radially outer surface tapered towards the rear end of the bush and adapted to engage radially the lip portion of the bush from the inside of the lip portion and push it against a radially inner surface of said base, in such a way as that said lip portion is adapted to exert a sealing contact against said radially inner surface of the base when said valved fluid path is in closed configuration and fluid pressure in the working chamber is greater than a fluid pressure in the accumulation chamber. a tubular base; a piston assembly comprising a piston mounted within the tubular base with an alternate sliding motion, said piston defining within the tubular base a working chamber and an accumulation chamber arranged on the rear side and on the front side of the piston respectively, and a stem connected to the piston; a plurality of fluid paths for connecting the working chamber with the accumulation chamber, at least one of which establishes a permanent fluid communication between the working chamber and the accumulation chamber; and valve means arranged on one of said fluid paths, hereinafter valved fluid path, and comprising a ring-shaped obstructing member arranged around the piston and capable of sliding along an axial length of the piston, wherein said valved fluid path comprises a path portion formed between the piston and the obstructing member, said valved fluid path being thereby capable of selectively assuming an open configuration, in which said valved fluid path establishes a fluid communication between the working chamber and the accumulation chamber, and a closed configuration in which said valved fluid path is blocked; wherein said obstructing member comprises a bush of plastic material, which has a flexible lip portion extending from a rear end of the bush and having a radially outer surface tapered towards said rear end of the bush; said shock absorber being characterized by further comprising a support ring member arranged coaxially with the bush and movable with respect thereto, said support ring member comprising a radially outer surface tapered towards the rear end of the bush and adapted to engage radially the lip portion of the bush from the inside of the lip portion and push it against a radially inner surface of said base, in such a way as that said lip portion is adapted to exert a sealing contact against said radially inner surface of the base when said valved fluid path is in closed configuration and fluid pressure in the working chamber is greater than a fluid pressure in the accumulation chamber.