The invention relates to a sachet (1) for foodstuffs, said sachet (1) being intended to contain a quantity of foodstuffs and to be introduced into a heating fluid in order to cook or heat said quantity of foodstuffs, said sachet (1) comprising a chamber (4) which contains said quantity of foodstuffs, and at least one region (3) which is permeable to said heating fluid, allowing both said heating fluid to come into contact with said quantity of foodstuffs in order to cook or heat the latter and also to create, inside said sachet (1), a flow of fluid vapour from the heating fluid during cooking or heating of the quantity of foodstuffs, said sachet (1) being characterized in that it comprises at least one region (5) which is impermeable to the heating fluid in order to at least partially retain the flow of fluid vapour inside said sachet (1). Sachet for cooking or heating a quantity of a foodstuff.