Prison spokeswoman JoEllen Smith is cited as saying that Ariel Castro was found hanging in his cell at 9:30pm (2:30am UK time) last night.
Prison medical staff performed CPR before he was transported to a hospital, where he was pronounced dead, she said.
Castro was sentenced to life in prison plus 1,000 years after pleading guilty to 937 counts including kidnapping and rape.
More top news
What is Debenhams' biggest shareholder doing, talking the retailer down in the crucial week before Christmas?
Knowing her and knowing them it is more likely they find a way to prolong the cancerous uncertainty for some weeks yet.
'Beryl received particularly nasty injuries in this incident', said Detective Sergeant Alex Wilkinson, of Greater Manchester Police.