Governor Newsom took office in January and a death penalty moratorium was one of his campaign pledges.
"The death penalty is inconsistent with our bedrock values and strikes at the very heart of what it means to be a Californian", Mr Newsom will say, according to prepared remarks from his office.
"The intentional killing of another person is wrong".
In addition to the moratorium, Newsom's order will also withdraw California's legal injection protocol and close the execution chamber at San Quentin, where all death row inmates are imprisoned.
Gavin Newsom announced a moratorium on the death penalty Wednesday, halting more than 700 executions in the state.
Newsom's office said 25 condemned inmates have exhausted all of their appeals and could have faced execution if the courts approved the state's new lethal injection method.
"The voters of the State of California support the death penalty", said Michele Hanisee, president of the Association of Deputy District Attorneys.
In his speech on Wednesday, the governor will highlight the racial disparities in sentencing - noting that 6 in 10 prisoners on California's death row are people of color - the cost of enforcing the death penalty and the number of innocent people who have been sentenced to death. According to an NAACP-LDF report, of around 740 prisoners on death row in California, 36 percent are black, though black people make up less than 7 percent of the state's population.
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Still, Newsom argued opposing the death penalty is an ethical question. His administration argues that capital punishment has been a failure, pointing at pervasive inequality running through the USA criminal justice system, the significant number of innocent people who have been wrongfully convicted, and evidence that the costly system doesn't increase safety.
Former Democratic Gov. Brown also opposed the death penalty, but his administration moved to restart executions after voters acted in 2016 to allow the use of a single lethal injection and speed up appeals.
California's governor is set to issue a moratorium on capital punishment in the US's most populous state, providing a reprieve for hundreds of inmates sentenced to death.
California hasn't executed anyone since 2006, when Arnold Schwarzenegger was governor.
Stanislaus County District Attorney Birgit Fladager, president of the California District Attorneys Association, also criticized Newsom for circumventing the will of a majority of voters. The governor's clemency powers are created to correct individual cases of injustice, said Scheidegger, legal director for the Criminal Justice Legal Foundation. Jerry Brown over their opposition to the death penalty.
The governors of Pennsylvania, Washington and OR have imposed moratoriums on executions, and the former Colorado governor refused to execute someone on death row.
Other governors also have enacted moratoriums. Richard Allen Davis was later sentenced to death for kidnapping, raping and murdering her. A repeal would require a popular vote in favour of the change.
California has not executed anyone in more than a decade because of legal challenges to the state's execution protocol.