The thermal bag (2) is made of thermally insulating composite material folded to form a bottom and welded along lateral borders (3) to form a body (1) of the bag with a mouth (5) at the level of which a handle (7A, 7B) is applied. The composite material defines a bottom folded in an accordion-like fashion opposite the mouth of the bag, and a laminar stiffening element (15) of the accordion-like bottom is disposed inside the bag.