The inmate, identified as 32-year-old Billy "Guero" Sell by an inmate advocacy group, was found dead in the segregation unit at the California State Prison near Corcoran July 22, The New York Times reported.
State officials did not confirm the death until Saturday when questioned by the Times, the newspaper said.
Spokeswoman Terry Thornton said the inmate, who was serving a double life sentence for attempted first-degree murder, was found unresponsive in his cell, and pronounced dead in the prison's hospital.
Laura Magnani, with American Friends Service Committee in San Francisco, said other inmates reported Sell had sought medical attention before his death, but was turned down.
"This is extremely disturbing news, not only because someone is dead, but because he had tried to get medical help and had been on hunger strike," Magnani said. "The complacency of the department about the seriousness of the prisoners' commitment, and the prisoners' desperation about their plight is shocking."
More than 630 inmates are participating in the hunger strike in nine California state prisons. The protest, which began July 8, is aimed at putting a five-year limit on how long inmates are held in isolation.