He was transported to an outside hospital and was listed in stable condition.
Sirhan, 75, is incarcerated at Richard J. Donovan Correctional Facility in San Diego.
He was stabbed Friday around 2:21 p.m.
Paramedics say they responded to the prison at that time for a male patient who had been stabbed in the neck and was bleeding and transported him to Scripps Mercy Hospital in San Diego.
He was described as being in stable condition.
"The suspect in the attack has been identified, and has placed in the prison's Administrative Segregation Unit, pending an investigation," the California Department of Corrections and Rehabilitation said.
Sirhan fatally shot Kennedy on June 5, 1968 as the New York senator was campaigning for president in Los Angeles at the Ambassador Hotel.
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Sirhan was later convicted of murder and sentenced to death. That sentenced was later changed to life in prison when California ended the death penalty in the 1970s.
He has applied for parole at least 15 times and been denied.