Spain's city transport systems don't follow one nationwide pet policy, so the rule that applies depends on which city you're in. Madrid's Metro and EMT buses, and Barcelona's Metro and bus network (TMB), both allow a small pet to travel in a closed, secure carrier as a matter of course, generally without an extra ticket, the same pattern found in most major European transit systems.
A larger dog travelling on a leash rather than in a carrier is the point where city rules diverge: some networks allow it with a muzzle and off-peak time restrictions, others are more relaxed, and it's worth checking the specific transit authority's current page for the city you're visiting rather than assuming one city's rule applies to the next. A PPP dog, regardless of city, must meet Spain's national muzzle and short-leash requirements on any public transport.
Assistance and guide dogs have the strongest, most consistent right of access: Spain's 2025 royal decree on assistance dogs guarantees access to public transport nationwide, without an extra ticket or seat charge, overriding any city-specific restriction that might otherwise apply to an ordinary pet.