Gov. Steve Sisolak on Tuesday issued the directive in accordance with new recommendations from the Centers for Disease Control. Mask use is mandated for all, regardless of vaccine status.
The mandate will go into effect at 12:01 a.m. on Friday.
The rules are in accordance with Sisolak's Emergency Directive 045, signed in May, to automatically adopt CDC guidance related to mask-wearing.
Based on CDC data, 12 of Nevada's 17 counties are deemed as having substantial or high transmission, including Carson, Churchill, Clark, Douglas, Elko, Esmeralda, Lincoln, Lyon, Mineral, Nye, Washoe and White Pine, the governor's office said.
"This update comes as the Delta variant now accounts for the majority of new cases. The Delta variant has shown to be much more contagious and spreads more than twice as easily from one person to another, compared with earlier strains," his office said in a statement.
The Nevada Gaming Control Board issued guidance on Tuesday telling all licensees in the high-transmission counties must have dedicated signage throughout the establishment notifying patrons where face coverings are required.
Both agencies urged residents to get vaccinated. "While using masks correctly has proven to be effective in helping to prevent people from getting and spreading COVID-19, the best most effective step people can take to protect themselves is to get fully vaccinated," the health district said in a statement.