The city council explains that the rule is part of a drive to clean up local beaches, stating that more portaloos will be installed in areas that currently lack toilet facilities.
Vigo's beach bylaws also include fines for practices such as using soap in water on the beach or in facilities for washing such as showers or footbaths, throwing waste of any kind on the sand, or using grills, gas cylinders or other fire hazards.
But there is scepticism over whether the northwest Spanish city would be able to enforce the ban on people using the sea to relieve themselves.
"How easy it is to issue prohibitions and how difficult to enforce them", commented Twitter user Pedro L Sierra.
Spanish beach resorts introduce new bans in bylaws every summer aimed at improving the collective experience.
Bare-chest ban in some parts of country
In 2014, Torrox became the first of a series of Costa del Sol resorts to ban the practice of leaving towels and parasols on the beach to bag a good spot, with police on quads patrolling and seizing unattended possessions.
Spain has this year been clamping down on beachgoers who fail to change out of their bikinis or swimming trunks while walking on the street. Being bare-chested has also been banned in some parts of the country.
"Some local councils will impose fines if you're caught wearing swimwear on the seafront promenade or the adjacent streets," UK government travel advice states.
