The new mandate from the Los Angeles County Department of Public Health requires people to wear masks indoors, even if they are already vaccinated against the disease.
But Los Angeles County Sheriff Alex Villanueva said the order is "not backed by science," and that it contradicts guidelines from the U.S. Centers for Disease Control and Prevention, which does not prescribe masks indoors for most vaccinated people.
The sheriff added that he "will not expend our limited resources" and that his office instead asks for "voluntary compliance" from the public.
The county, home to 10 million people and the nation's second-largest city, Los Angeles, is grappling with COVID-19, its Delta variant and case numbers rising to worrisome levels.
The DPH announcement comes after six straight days of more than 1,000 new COVID-19 cases reported in Los Angeles County.