California Governor Gavin Newsom said he'll change all death sentences in the state to life in prison without the possibility of parole.
He made the announcement as he prepares to leave office after eight years.
There are currently 548 prisoners on death row in California, the largest system of its kind in the Western Hemisphere.
Newsom said in a letter announcing the move: "I take this action because I firmly believe that state-sanctioned killing is incompatible with California's values, undermines the foundations of our legal system, and diminishes each of us."
In 2019, Newsom placed a moratorium on executions in California but hundreds of people remained on death row - 529 of whom are men and 19 are women.
He called the latest decision one of the most difficult choices of his career, given it will trigger painful memories for crime victims' families.
Newsom - who is expected to seek his party's presidential nomination in 2028 - went on: "As governor, I alone bear responsibility for this decision and its impact, just as I alone would bear responsibility for each execution."
The American Civil Liberties Union welcomed the announcement.
Its executive director Anthony Romero said in a statement: "These commutations reflect what justice demands: an end to a racist, error-prone, and inhumane punishment that has burdened California for far too long."
The ACLU previously challenged California's death penalty statute, calling it racially discriminatory and unconstitutional.
Newsom said in the past that capital punishment discriminated against people of colour and the mentally ill.
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In September, convicted murderer Christa Pike survived two lethal injections in Tennessee after spending three decades on death row.
The 50-year-old American was given two doses of pentobarbital, a lethal drug used by the US state in executions.
State governor Bill Lee has ordered an investigation into what happened during the failed execution and suspended all remaining scheduled executions this year.
The death penalty in still in place in 27 US states, though executions are concentrated in a handful.
More than 20 states across the country have abolished capital punishment altogether.
(c) Sky News 2026: All 548 death row inmates in California to have sentences changed to life in prison

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