The invention seeks to enable an accessory to be attached in modular and safe manner to a mobile unit while providing a choice between at least two fastener means. An accessory attachment system for a mobile unit in accordance with the invention, in which the accessory has an anchor point for a ring on which there is mounted a loop of cord for connection to the mobile unit presents a plate suitable for being secured to the shell of the mobile unit, and at least one O-ring of elastic material suitable for sliding or rolling along the external antenna in order to take up a clamped position at the root of the antenna, the plate and the O-ring being mounted on the cord. Advantageously, the accessory and the plate are mounted on the cord via respective key rings.