A digital certificate issued by the FNMT (Fábrica Nacional de Moneda y Timbre) that lets you sign documents and authenticate yourself on Spanish government websites. Essential for TIE renewal via Mercurio.
Request online at sede.fnmt.gob.es, get a pre-confirmation code, attend an in-person verification at an authorized office (such as the AEAT tax office), then download and install the certificate on your computer.
Forms occasionally get revised. Always download a fresh copy on the day you plan to use it.