It is the most restrictive order by a governor so far during the novel coronavirus pandemic.
It applies to most of the state's 40 million residents. There are exceptions for workers in "16 critical infrastructure sections."
The order takes effect Thursday night.
"Those that work in critical sectors should go to work. Grocery stores, pharmacies, banks and more will stay open," Newsom tweeted. "We need to meet this moment and flatten the curve together."
Law enforcement will not arrest violators, Newsom said in a news conference today.
"I don't believe the people of California need to be told through law enforcement that it's appropriate just to home isolate, protect themselves," he told reporters.
The governor said the order is open-ended. "This is a dynamic situation," Newsom said.
It comes as the number of cases in California and the United States skyrocket.
The rising toll has medical officials around the country looking at how to treat the deadly virus, but one of the nation's preeminent infectious disease experts told CNN on Thursday that "there's no magic drug" now.
The number of reports of positive tests has gone from a few thousand on Sunday to more than 13,000 as more people are infected and more people with Covid-19 find out through testing they have the disease.
In a news briefing Thursday, President Donald Trump said he had pushed the US Food and Drug Administration to eliminate barriers to getting therapeutics to coronavirus patients.
Trump said the antimalarial drug chloroquine and its analog hydroxychloroquine would be available by prescription to treat the novel coronavirus.
Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said the two drugs would be made available but there needs to be way to see how safe they are and whether they work to inhibit coronavirus.
"What the President was saying is that we're going to look at all of these drugs and we're going to try to get them available in the context of some sort of protocol," he said, referring to a trial period, on CNN's "Coronavirus: Fact and Fears" town hall special.
The doctor wanted to make sure Americans know there are no proven safe and effective therapies as of today.
"That doesn't mean that we're not going to do everything we can to make things that have even a hint of efficacy more readily available," Fauci told CNN Chief Medical Correspondent Dr. Sanjay Gupta. "But there's no magic drug out there right now."