Sirhan Bishara Sirhan was approved for parole by a two-person born in August but Mr Newsom has stepped in to prevent his release.
Robert F Kennedy Jr and Douglas Kennedy had supported Sirhan’s release, but it was opposed by other family members.
Sirhan has spent 53 years behind bars for Kennedy’s 1968 killing at the Ambassador Hotel in Los Angeles following a presidential campaign event.
Kennedy, 42, had been celebrating his victory in the California primary and died a day after being shot.
His killer was initially sentenced to death for the killing, which was commuted to life imprisonment in 1972 when the state’s supreme court ruled the death penalty unconstitutional.
“After carefully reviewing the case, including records in the California State Archives, I have determined that Sirhan has not developed the accountability and insight required to support his safe release into the community. I must reverse Sirhan’s parole grant,” Mr Newsom wrote in an editorial in The Los Angeles Times.
Mr Newsom added that Sirhan, 77, still “remains a potent symbol of political violence.”
“He does not understand, let alone have the skills to manage, the complex risks of his self-created notoriety,” he added.
“He cannot be safely released from prison because he has not mitigated his risk of fomenting further political violence.”
It was the 16th time that Sirhan had tried to secure his release on parole.
Following Mr Newsom’s decision, members of the Kennedy family issued a statement thanking him for his action.
“Instead of contrition, this inmate points to what he sees on the clock rather than to what he knows in his heart, believing somehow that the passage of time is expiation enough,” their statement said.
“It is not enough, and no time-served is long enough to justify paroling a man entirely lacking insight into his premeditated political assassination.”