Beach access for dogs in Spain has no single national rule — it's set municipality by municipality, through each town's own ordenanza (local ordinance). The common pattern, confirmed in coverage of cities drafting these rules, is that dogs are generally barred from the main sand year-round, with access limited to beaches specifically designated as a playa canina (dog beach), or to an off-season window at ordinary beaches, such as 1 November through 1 March.
Where a dog beach or off-season window exists, typical conditions include keeping the dog leashed (muzzled if needed), cleaning up waste immediately, a cap on the number of dogs per adult, and excluding aggressive or unsocialised animals — rules set at the municipal level, under sanctioning frameworks that themselves vary by autonomous community (Spain's Consumer organisation Consumer.es and reporting on specific towns both confirm this local-ordinance pattern).
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 town hall's own tourism page before you go. A dedicated dog-beach map exists for the current season in several media outlets, which is a reasonable starting point for planning a coastal trip, but always confirm against the specific town's current ordinance rather than relying on a general list alone.