Disclosed is a closure for ducts or receptacles for pressurized fluids, including an annular supporting structure (10), which defines an opening (11) for accessing the duct or receptacle, and a hatch (30), connected in an articulated manner to the supporting structure (10), with the ability to move between a closed position and an open position, wherein the supporting structure (10) includes a sealing surface (13), characterized in that it includes a sealing element (20) that can be positioned in the opening (11), the sealing element (20) including an expansible portion (21) with an inner surface (24) facing the duct and an outer surface (23) facing the sealing surface (13), the expansible portion (21) being able to expand as a result of the pressure of the fluid that acts on the inner surface (24) so that the outer surface (24) is placed in contact with the sealing surface (13).