Theofilos Sechidis, who shocked Greece with the quintuple murder of his family in the 1990s, was found dead on Tuesday at the psychiatric ward of Korydallos prison.
According to information, the 46-year-old convict died while he was in a shower. No further details were immediately available on the cause of death.
In May 1996, the then 24-year-old, law school student Theofilos Sechidis confessed to murdering his parents, sister, grandmother and an uncle on the northeastern Aegean island of Thasos.
He told police he murdered each person separately over a five-day period, hacking up their bodies and placing the remains in black plastic garbage bags.
Asked why he did it, he said his family were plotting against him. A psychiatric evaluation at the time concluded he suffered from schizotypal disorder.
On June 1997 he was found guilty of five counts of murder and convicted to life imprisonment.
Sechidis had submitted a request for his release in 2016, which had been turned down, while a second one was pending.