Aboubacar Sidiki Diakité - also known as Toumba - died early on Wednesday in hospital following a medical emergency, the prison administration said. BBC reported.
He was serving a 10-year sentence for his role in one of the darkest episodes in Guinea's history in which more than 150 people were killed in the capital, Conakry, as they demanded an end to military rule. At least 109 women were also raped.
While in custody, Diakité remained a controversial figure, creating a political party and attempting to run for president.
Last year, he declared his intention to stand in the presidential elections, which was rejected by the Supreme Court after it ruled he was ineligible.
At the time of the massacre, Diakité was commander of the presidential guard and a close ally of Guinea's then military ruler, Moussa Dadis Camara.
