The statewide order, which Governor Gavin Newsom said goes into effect "this evening," is the most dramatic containment measure yet by a US state.
"There is a mutuality, there is a recognition of our interdependence, that requires of this moment that we direct a statewide order for people to stay at home," he said.
No end date was set for the measure because the pandemic is a "dynamic situation," said Newsom.
California is home to more than 39 million people, and has been among the worst hit by the pandemic.
It has experienced at least 18 deaths from the coronavirus, according to a Johns Hopkins tally.
Newsom said he did not expect law enforcement would be required to enforce the measure.
"People I think recognize the need to do more, and to meet this moment, and people will self regulate their behavior," he said.
Essential jobs and tasks such as grocery shopping would be exempt, he said.
"We want people to go to essential jobs -- we want people to deal with their essentials in a thoughtful and judicious way." In a letter to US President Donald Trump Wednesday requesting federal help, Newsom warned that cases were doubling every four hours in some parts of the state.
"We project that roughly 56 percent of our population -- 25.5 million people -- will be infected with the virus over an eight week period," he wrote.
Newsom's announcement came moments after Los Angeles Mayor Eric Garcetti imposed a similar order on the state's largest city.