Tesla Inc. is telling workers at its Nevada battery factory they will be required to wear a mask indoors starting Monday regardless of vaccination status, joining the growing number of companies with such mandates as Covid-19 cases rise.

Workers at the Reno, Nev.-area facility had previously been required to wear a mask if they weren’t vaccinated, but could avoid that requirement if they were, people familiar with the matter said. The company cited the spread of the highly contagious Delta variant of the virus and updated Centers for Disease Control and Prevention guidance in its policy change.

Tesla didn’t respond to a request for comment. The Nevada government has said it may impose a mask mandate this week in the county where Tesla’s factory is located if transmission rates don’t improve.

Workers at the Nevada factory will now have to wear a mask indoors regardless of vaccination status.

Photo: Rich Pedroncelli/Associated Press

Counties in the San Francisco Bay Area, where Tesla has an auto plant, mandated that people wear a mask indoors starting last week.

Tesla’s move follows a similar one by its Detroit car company rivals, which said last week that they were reinstating a mask mandate for workers at all factories, offices and warehouses.

Tesla Chief Executive Elon Musk last year struck a defiant tone in the early days of the coronavirus outbreak, battling to reopen his U.S. car plant, which was temporarily shut down as local authorities tried to stem the spread of the virus. Months later, Mr. Musk said he tested positive for the virus. Despite the weekslong disruption, Tesla last year delivered its first full-year of profit.

Write to Rebecca Elliott at rebecca.elliott@wsj.com