Unlike the entry and rail rules covered elsewhere in this guide, beach access for dogs in France has no national rule at all. Each commune decides for its own beaches through a municipal order (arrêté municipal), which means the rule at one beach can be completely different a short drive down the coast.
The common pattern, where a restriction exists, is a seasonal one: many communes ban dogs from supervised (surveillance) beaches during the summer season, when crowds peak, while allowing them year-round on unsupervised stretches, or restricting hours to early morning and evening during the summer months. Some towns designate a specific dog beach as a dedicated alternative.
Because this is set locally, the only reliable way to know the rule for a specific beach is to check the signage at the entrance or the commune's own tourism office or town hall website before you go — a rule that applied on a previous trip to a different stretch of coast may simply not apply here.