Ariel Castro has been found dead in his cell after being sentenced to life plus 1,000 years in prison for kidnapping and raping Amanda Berry, Michelle Knight and Gina DeJesus.
Prison officials said he died in hospital late on Tuesday, after prison medical staff failed to revive him.
Castro, 53, held the three women captive in his Cleveland home for more than a decade.
He kept his victims chained up and raped them.
A spokeswoman for the Ohio Department of Rehabilitation and Correction said Castro was placed in protective custody because of the notoriety of his case, but was not on suicide watch.
JoEllen Smith, said Castro was was in a cell by himself, and rounds were conducted every 30 minutes at staggered intervals.
Castro had pleaded guilty to 937 counts, including murder and kidnapping, in exchange for the death penalty being taken off the table.
During the sentencing, Castro denied being abusive and insisted that most of the sex was consensual.
"I'm not a violent person," Castro said. "I simply kept them there without them being able to leave."
The three women disappeared separately between 2002 and 2004. They escaped on May 6, when one of the women broke part of a door and yelled to neighbours for help. Castro was arrested that same night.
In a victim impact statement made to the sentencing court, Ms Knight told how their captor went to church every Sunday, before coming home to "torture" the women.
"I spent 11 years in hell. Now your hell is just beginning," she said. "You will face hell for eternity.
"From this moment on, I will not let you define me, or affect who I am. I will live on, you will die a little every day."
Ohio State Police and the Department of Corrections are investigating the death.