A device for coupling together in leaktight manner two smooth tubes disposed end-to-end, the device comprising a sleeve which diameter can be reduced by a tightening action, and a sealing ring (24) suitable for being disposed inside the sleeve so as to surround the ends of the two tubes. The ring is constituted by a looped flat strip (26) having two longitudinal ends (26a, 26b) suitable for coming into contact with each other when the sleeve is tightened around the tubes (20, 22). The first longitudinal end of the ring is provided with a notch (28), while the second longitudinal end of the ring is in the form of a tongue (30) suitable for being inserted into the notch. Both the width of the tongue and also the width of the notch taper in the direction going along the strip from the first longitudinal end to the second longitudinal end. When the tongue is in a partial insertion position in which it is inserted partially in the notch, the two sides (30a, 30b) of the tongue are suitable for coming into contact with the two sides (28a, 28b) of the notch while the free end (30a) of the tongue remains at a distance from the end wall (28c) of the notch, and, starting from the partial insertion position, the tongue is suitable for penetrating further into the notch under the effect of the tightening.