Sammeon Waller, 28, is accused of killing 66-year-old John Sullivan, who was found unresponsive in his cell early Wednesday morning at San Quentin State Prison, according to correctional officials.
Sullivan was admitted from Placer County on March 3, 2016, to serve a 10-year sentence for failure to register as a sex offender, a second-strike.
He was pronounced dead shorty after 1 a.m., authorities said.
Investigators later identified Waller as the suspect.
Waller was admitted from Los Angeles County on Dec. 20, 2017, to serve seven years for first-degree burglary and injury after his conviction in the attack on his 82-year-old grandfather in Pomona.