An implantable vesical prosthesis for a bladder which includes a sterile pouch formed from flexible plastics material compatible with the organs surrounding the natural bladder of a patient, at least a portion of the pouch being deformable during the introduction and discharge of urine between two extreme positions, one corresponding to the full pouch and one to the empty pouch. Two inlet pipes open into the interior of the pouch and are connectable to two ureters of a patient and an outlet pipe is connectable to the urethra. The prosthesis is completely void of any internal artificial valves or flaps.