Abstract: A coupling for tubes with a body with a neck (14) provided with two or more longitudinal recesses (15) and each of which is positioned a jaw (12) able to move radially between an expanded release position and a contracted lock position. The jaws can move from one position to the other by means of cam surfaces (24) provided inside a tightening nut (13) assembled and susceptible to partial rotation on the neck of the coupling body. Furthermore the nut has means for interacting with the neck to block it and prevent it from turning in the release direction.