California will once again require residents to wear masks in indoor public settings everywhere in the state, amid uncertainty surrounding the rapid spread of the Omicron coronavirus variant and rising case rates as the holidays approach.

The mandate will go into effect on Wednesday and will remain until at least Jan. 15, state officials said on Monday. The state will also require unvaccinated people attending so-called mega-events to show proof of a negative coronavirus test result from within a day, if it’s an antigen test, and within two days for a P.C.R. test, and formally recommended that travelers returning to California get tested within a few days of their arrival.

“We know people are tired and hungry for normalcy,” Dr. Mark Ghaly, the state’s secretary of health and human services, told reporters.

But he said that science has shown that even a relatively small increase in indoor masking can help prevent the virus from spreading, which can in turn help keep hospitals from filling.

Dr. Ghaly said that statewide, case rates have increased by 47 percent since Thanksgiving, but those increases have been concentrated in parts of the state — such as the Inland Empire, east of Los Angeles, and the vast Central Valley — where a smaller share of the population has been vaccinated.

Those are, by and large, the same places where mask mandates have not been in effect for some time. In other areas, such as Los Angeles County, residents have been required to wear masks indoors for months since the emergence of the Delta variant.

For Californians, the imposition of a fresh round of pandemic restrictions in mid-December is an unwelcome echo of last year’s catastrophic winter.

Last December, officials in the state — where residents had already been living under some of the most stringent lockdowns in the country for the better part of a year — imposed a curfew and new stay-at-home orders in a frantic effort to keep hospitals from being overwhelmed.

Dr. Ghaly said that the mask mandate was meant to head off the need for any further restrictions, and he urged Californians to adhere to it, even in the absence of strict enforcement.

“Part of what we’re doing today is making sure those conversations stay off the table,” he said, referring to other measures, such as business capacity limits. “I think that we have tools to keep California reasonably safe and protected.”

On Monday, New York State began requiring people wear masks indoors in public if proof of full vaccination was not required, a policy that is in effect at least until Jan. 15. Businesses that do not require patrons to be masked could face civil and criminal penalties, including fines of up to $1,000 per violation, and local health departments are responsible for enforcement.

When asked whether other states should follow California’s actions, Jen Psaki, the White House press secretary, said Tuesday that “we really leave it to leaders in states to make decisions about what needs to be done to keep communities safe.”

Giulia Heyward contributed reporting.