Zahara de los Atunes, near Cadiz, used tractors to spray more than 2km (1.2 miles) of beach with a bleach solution a day before Spain allowed children out of lockdown for the first time.
Environmentalists say the move caused "brutal damage" to the local ecosystem.
Spain has been badly affected by the coronavirus, with 23,800 deaths.
It recently announced a four-phase plan to lift its stringent lockdown measures and return to a "new normality" by the end of June.
