Abstract: The invention refers to a seal of guarantee (10) for foodstuff wrapped in a packaging, which closes said packaging and which has the shape of a label, having a tab (11) which can be folded down on to itself to cover a sufficient portion of the packaging, with a zone (12) where it is possible to write information regarding the product, such as the production and best before dates. The invention also refers to a machine to automatically apply the seal (10) which fundamentally includes a loader (17) containing a pile of seals, which are picked up one at a time from the underside, a cursor (19) which handles the seals to be applied, a tool (21) to fold the tab (11) and a seal applicator head (18) for ultrasonic welding, gluing or similar.