Ariel Castro, who kept women captive at his home in Cleveland, Ohio, has died after being found hanging in his cell.
Prison officials said he died in hospital late on Tuesday, after prison medical staff failed to revive him.
Castro, 53, held three women against their will for about a decade until May this year. He chained the women, beat and raped them.
The prosecutor who tried him called him a "coward" unable to withstand "a small portion" of what he had inflicted.
Castro was sentenced on 1 August to life imprisonment without parole plus 1,000 years. The house was demolished.
The former school bus driver abducted Michelle Knight, 32, Amanda Berry, 27, and Gina DeJesus, 23, from the Cleveland streets between 2002-04.
A spokeswoman for the Ohio Department of Rehabilitation and Correction, JoEllen Smith, said: "He was housed in protective custody which means he was in a cell by himself and rounds are required every 30 minutes at staggered intervals.
"Upon finding inmate Castro, prison medical staff began performing life saving measures. Shortly after he was transported to [the prison medical facility] where he was pronounced dead at 10:52 pm."
"A thorough review of this incident is under way," she added.
A preliminary medical examination found his cause of death was hanging, said Dr Jan Gorniak, the Franklin County coroner. He used a bed sheet.
Cuyahoga County Prosecutor Timothy McGinty spared no words for Castro in a statement following his apparent suicide.
"These degenerate molesters are cowards," Mr McGinty said. "This man couldn't take, for even a month, a small portion of what he had dished out for more than a decade.
"Let this be a message to other child kidnappers: There will be a heavy price to pay when you are caught. You won't enjoy the captive side of the bars."
Castro was placed in protective custody because of his notoriety, but was not on suicide watch, which requires constant observation.
A former neighbour said Castro's death had cut short the life term called for in his plea deal.
"It does give a little bit of closure to the families and people that got affected by what he did, but at the same time he deserved to be in there for his life because of what he did to those girls," Jessica Burchett told the Associated Press news agency.
Gina DeJesus was 14 years old when she disappeared. Amanda Berry was 16, and Michelle Knight, 21. Castro fathered a child with Amanda Berry.
The three women escaped from Castro's home on 6 May, when Amanda Berry broke part of a door and yelled to neighbours for help.
Castro pleaded guilty to the 937 charges against him, including multiple counts of kidnapping, rape and aggravated murder in connection with the beating of a pregnant captive until she miscarried.
A deal with prosecutors spared him a possible death penalty.
At his sentencing, Castro told the court that he had been "driven by sex", adding: "I'm not a violent predator… I'm not a monster, I'm a normal person.
"I'm just sick. I have an addiction, just like an alcoholic has an addiction."
He said he never planned to abduct the women but acted on the spur of the moment when he kidnapped his first victim. He said that he was "truly sorry" for what he had done.
During the sentencing hearing, Judge Michael Russo told Castro there was no place in the world for people who enslave others.
At that hearing, Ms Knight said Castro 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."
She was the only victim to speak in person at the hearing.
In an interview last month after his conviction, Castro's lawyers said that he fit the profile of someone with a sociopathic disorder. They expressed hope that researchers would study him for clues that could be used to stop other predators.