The executive order grants a reprieve to 737 inmates on the country's largest death row and halts the use of the death penalty in the state, according to the governor's office.
During an exclusive sit-down interview with ABC affiliate station KGO, Newsom called the decision "personal" and "moral" for him, while also pointing to what he sees as systemic racial and economic inequalities in the criminal justice system.
"I cannot in good conscious sign death warrant for someone," Newsom told ABC’s Eric Thomas from the governor’s mansion. "This is coming on me... I just can’t do it."
Newsom has long been an opponent of the death penalty, but he said in the interview that the question was largely academic until he was elected governor. Now, he would be asked to approve a death penalty protocol in the state, and he says it is "raw and emotional."
"This is not a quick response," he went on explaining his decision. "This is not a political response… this is a deeply held belief."
"I have to live with myself," he continued, but acknowledged that he thought "good people" would completely disagree with him on this issue.
Citing criminal justice figures in his state and around the country, Newsom added he thought that poor Americans and racial minorities are too often treated differently under the law.
"The American justice system is completely broken, and we all are perpetuating it. The systemic racism, the implicit bias, the overt bias, the whims of prosecutors based on geography, based on the will of people in the moment, fear and anxiety... until we address that I don’t think we can do what Saudi Arabia is doing and what North Korea is doing," he added.
Early Wednesday morning, Trump tweeted his dismay at the governor's move, expressing solidarity with the family and friends of victims.
The decision is especially controversial considering that California voters rejected a ballot measure that would have repealed the death penalty in 2016, but Newsom said he was elected for his judgment.
Newsom, who signaled that he wanted to move on the issue when he first took office, cited high costs, racial inequities and lack of deterrent as key reasons behind the decision.
"The death penalty is absolute. Irreversible and irreparable in the event of human error," he added, noting that the state had spent $5 billion since 1978 to keep inmates on death row and that across the country more than 100 citizens on death row have been found innocent.
"I do not believe that a civilized society can claim to be a leader in the world as long as its government continues to sanction the premeditated and discriminatory execution of its people," Newsom added.
"If you rape, we don’t rape you back," he went on during the interview. He called the entire American justice system broken.
"In short, the death penalty is inconsistent with our bedrock values and strikes at the very heart of what it means to be a Californian," he said.
Newsom, a Democrat, said he is not issuing commutations for the convicted and that he does not believe life sentences should be changed.
California's last execution was in 2006, under Gov. Arnold Schwarzenegger.
The executive order will also close the execution chamber at San Quentin State Prison, which has never been used, and withdraws California’s lethal injection protocol
Sharon Sellitto, a victim’s advocate whose brother, Paul Cosner, is believed by authorities to be a murder victim of serial killers Charles Ng and Leonard Lake, told ABC she is "heartbroken" by the governor’s decision.
"He's not the judge, not the jury and was not at the trial," Sellitto told ABC News in a phone interview Tuesday evening. "He should be concerned with the victims, not the perpetrators."
In 2001, a San Francisco Superior Court judge officially ruled that Cosner "was the victim of murder at the hands of Leonard Lake and Charles Ng."
Sellitto said she received a phone call Tuesday night from the Department of Corrections Office of Victims Survivors Rights and Services giving her a heads-up about the governor's announcement and providing her with a contact in his office if she wished to speak to anyone further about his decision.
"Awful, just awful," she said. "Nobody should use the word 'justice' in my presence again."
Mike Semanchik, managing attorney for the California Innocence Project -- which works to exonerate inmates on death row -- cheered the move.
"Conservative estimates suggest 4 percent of people on death row are innocent. That conservative estimate means 29 of the 737 people are awaiting execution for a crime they did not commit," Semanchik tweeted Wednesday. "Thank you, Gavin Newsom, for eliminating the risk of executing the innocent!"
"It has been my dream for many years that we would end the human rights violation known as the death penalty in California," Justin Brooks, director of the California Innocence Project and a Professor at California Western School of Law, said. "It is certain that as long as there is the death penalty there is the risk of executing innocent people. I am proud of our new Governor for taking this bold step."
Kim Kardashian West, who has championed criminal justice reform and successfully lobbied President Donald Trump to commute a Tennessee woman's life sentence, said she was "very supportive" of the governor's decision.
Criminal justice experts said Newsom's decision will most likely be challenged in court.
The Association of Deputy District Attorneys, which represents about 1,000 deputy district attorneys in Los Angeles County, called the decision "hasty and ill-considered."
"The voters of the State of California support the death penalty," Association President Michele Hanisee said in a statement Tuesday amid rumors about the governor's decision. "Governor Newsom, who supported the failed initiative to end the death penalty in 2006, is usurping the express will of California voters and substituting his personal preferences via this hasty and ill-considered moratorium on the death penalty."