France's national parks are split into a protected "zone cœur" (core zone) and a more lightly regulated surrounding area, and the dog rules between the two are dramatically different. In the core zones of several major parks — Vanoise, the Écrins, the Pyrénées and Guadeloupe among them — dogs are banned outright, even on a lead or carried, with the ban at the Écrins specifically set out in the park's founding decree. Mercantour bans dogs in its core too, with limited exceptions for working dogs.
The reasoning is consistent across parks: dogs retain a predatory instinct that can stress wildlife even without any intent to harm, they can provoke conflict with livestock guardian dogs working in the same terrain, and they can carry parasites that affect wild and domestic grazing animals alike. A small number of parks take a more relaxed approach — the Cévennes allows dogs on a lead throughout, for instance — so it's genuinely worth checking the specific park you're visiting rather than assuming one rule applies everywhere.
Exceptions, where they exist, tend to be narrow: assistance dogs, working livestock guardian or herding dogs, dogs accompanying a legally authorised hunt, and dogs belonging to residents of hamlets inside the park boundary. For everywhere outside the core zone — which is most of a national park's total area — normal leash etiquette and the forest leash season covered elsewhere in this section still apply.