The invention relates to a gastroplasty ring formed by a flexible strip (2) for passing around the stomach and for being closed near its two end portions (5, 6) by a closure system (7, 12, 13, 15, 20) to reduce the diameter of the opening of the stoma, said strip having an annular compression chamber of adjustable volume and of substantially constant section that is connected by an adjustment catheter (7) to a device (8) for adjusting the internal pressure in said chamber so as to adjust its diametral expansion, the ring being characterized in that the closure system includes means (12, 13) for locking and loosening the ring, which means are carried by the adjustment catheter, thus making it possible, starting from the position in which the diameter of the ring is locked, to release the diameter of the ring by relative displacement between the two end portions. The ring constitutes a gastric implant for treating obesity.