Abstract: A tightening and sealing device is made from plastic material and includes two members which can be coupled to each other. The member acting as the tightening element includes an internally threaded hollow cylinder divided into three cylindrical segments by longitudinal and axial slots spaced from each other by 120.degree.. Each segment has extending outwardly from an end thereof and centered between adjacent slots a flexible leg. The legs are joined to the end of an outer body which is concentric with the cylinder and which is internally cylindrical and externally polygonal. The second member which acts as the sealing element is composed of a cap-shaped element having an axial bore and an annular flange dimensioned to fit into a space between the concentric bodies forming the tightening element.