Abstract: A hot water bag made of a soft, flexible, rubber-like synthetic material and defining two side walls of a substantially rectangular shape and having a filling neck at a shorter side thereof, comprising a soft flexible coat seam-welded to at least one of said side walls. The hot water bag is manufactured by injection die molding process in assembled outer molds including a core; the bag body with embedded filling neck is first manufactured with a side slit for removing the die core then the pliant coat is seam welded to one side and finally the slit between the side walls is closed by welding.