By Agence France-Presse
Staff at San Quentin State Prison found Andrew Urdiales, 54, unresponsive and alone in his cell late Friday and were unable to revive him, officials said in a statement.
Late on Sunday, the second inmate, Virendra Govin, 51, was also found dead, they added.
Both deaths are being treated as suicides, prison officials said, adding that there was no indication the two incidents are related.
Urdiales was sentenced to death on October 5 of this year for a string of murders dating back to as far as 1986. The killings took place while he was stationed at various US Marine Corps facilities in southern California.
He was placed on death row on October 12.
He had previously also been convicted and sentenced to death for several murders in the mid-1990s in the state of Illinois but that sentence was commuted in 2011.
Govin was sentenced to death in 2004 for killing four members of a family in Los Angeles County and then setting their house on fire.
He was placed on death row in 2005.
"Since 1978, when California reinstated capital punishment, 79 condemned inmates have died from natural causes, 25 have committed suicide, 13 have been executed in California, one was executed in Missouri, (and) one was executed in Virginia," prison officials said in a statement.
There are currently 740 inmates on California's death row, they added.