The mandate will take effect from Dec. 15 to Jan. 15, 2022, according to Dr. Mark Ghaly, secretary of the state Health & Human Services Agency.
Many counties across California, including Los Angeles, already have such mask mandates in place, Ghaly noted. The state rule will cover those counties that do not - representing about 50% of the state's population.
The new rules require the wearing of masks in all indoor public settings.
"Even a 10% increase in indoor masking can reduce case transmission significantly," Ghaly noted.
Ghaly said the state has seen a 47% increase in COVID-19 case rates across California since Thanksgiving, jumping from 9.6 cases per 100,000 Californians to now more than 14 per 100,000.
He said case rates in Los Angeles and the Bay Area remain relatively low, but other areas, particularly those with low vaccination rates, are seeing bigger increases and strained hospital capacity.
Areas where vaccine rates are low and hospitals are strained include Riverside and San Bernardino counties, the Central Valley, San Joaquin Valley and the northernmost parts of the state, Ghaly said.
"We are proactively putting this tool of universal indoor masking in public settings in place to ensure we get through a time of joy and hope without a darker cloud of concern and despair," he said.
The state is also tightening restrictions on unvaccinated people who attend large public events.
Any person attending an event with more than 1,000 people who cannot show proof of vaccination must now show proof of an antigen test within the previous 24 hours or a PCR test within the past 48 hours.
The current rule allows any test within the previous 72 hours.
Ghaly said that window is being tightened because more tests are available and they have faster turnaround times for results.
In addition, the state is recommending, but not requiring, that travelers who enter or return to California get tested within three to five days of their arrival.
Meantime on positive note, more Americans are getting booster shots as health experts continue to stress the latest research.
"When you get a booster, for example a third shot of an mRNA, it raises the level of protection high enough that it then does do well against omicron," said Dr. Anthony Fauci, the White House chief medical adviser.
Experts say that boosters, coupled with renewed efforts, are crucial to containing COVID spread.
"Californians have done this before, and we, of course, believe we can do it again," Ghaly said.
MORE: How severe is omicron? Expert says variant's 50 mutations could be its downfall