"The intentional killing of another person is wrong, and as governor, I will not oversee the execution of any individual", he plans to say.
Newsom's aides said it has not yet been decided what will become of the execution chamber, nor whether corrections officials have been told to top preparing for executions, for instance by running drills.
How did we get here?
Ultimately, Newsom decided the moratorium was in the best interest of the state, because he believes the death penalty "is inconsistent with our bedrock values and strikes at the very heart of what it means to be a Californian".
"Defying voters, the Governor of California will halt all death penalty executions of 737 stone cold killers", Trump tweeted. And though voters in 2016 narrowly approved a ballot measure to speed up the punishment, no condemned inmate faced imminent execution.
Ellen Kreitzberg, a death penalty expert and opponent at Santa Clara University law school in California, welcomed Newsom's move.
Gavin Newsom (D) has been a star in the Democratic Party.
Why did he do this?
Newsom has always been a vocal opponent of the death penalty, stating the practice discriminates against people of color, mentally ill people and poor people, and that it has put to death those wrongfully convicted of crimes. In remarks Wednesday, Newsom plans to note that 25 inmates on California's death row have exhausted their legal appeals and "could soon be eligible for execution". Thirty-six states have either abolished the death penalty, put executions on hold or not carried out an execution in at least five years. It will not affect inmates' convictions or their imprisonment, other than eliminating their death sentences, the source said.
California has the biggest death row population in the country, with one in four U.S. inmates on death row incarcerated in the state.
Capital punishment is considered a legal penalty in the State of California.
But Newsom's action will anger death penalty proponents.
An armed California Department of Corrections and Rehabilitation officer stands guard at San Quentin State Prison's death row.
Newsom said the death penalty isn't a deterrent, wastes taxpayer dollars and is flawed because it is "irreversible and irreparable in the event of human error". Voters shot down a proposed end to the death penalty by 4 points in 2014 and 6.4 points in 2016, reported Politico.
California has not put an inmate to death since 2006, amid legal challenges to its execution protocols and discomfort with the sentence among political leaders.
Newsom has recognized the cruelty, the moral and fiscal costs, and the discriminatory nature of death sentencing in the state.