Abstract: The invention relates to a device for the treatment of obesity or excess weight of a domestic or pet mammal, the device consisting of a balloon which is adapted to be placed in the animal's stomach and comprises a flexible envelope (E) sealed against air and alimentary and/or physiological liquids, as well as an inflating valve (V) hermetically sealed to the flexible envelope (E). The invention also relates to the use of a flexible envelope (E) sealed against air and alimentary and/or physiological liquids and equipped with an inflating valve (V) to form a device for the treatment of obesity or excess weight of a domestic or pet mammal, this device being designed to be placed in this animal's stomach and filled in-situ with fluid.