Castro was found hanging in his prison cell at 9 p.m. Tuesday at Correctional Reception Centre in Orient, Ohio, officials said.
Castro was not part of the general prison population and was kept alone, the officials said.
After finding Castro in his cell, prison staff tried to revive him, ABC News reported citing corrections spokeswoman JoEllen Smith.
Castro was taken to Ohio State University Centre and was declared dead at 10.52 p.m.
"Inmate Ariel Castro was found hanging in his cell this evening at 9.20 p.m. at the Correctional Reception Centre in Orient. 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," ABC News quoted Smith as saying in a statement.
"A thorough review of this incident is under way and more information can be provided as it becomes available pending the status of the investigation," the statement concluded.
The former bus driver, 53, was sentenced to life in prison plus 1,000 years in August this year for kidnapping Amanda Berry, Michelle Knight and Gina DeJesus and sexually assaulting and beating them while holding them captive in his Cleveland house for over a decade.
Castro had pleaded guilty to 937 counts, including murder, rape and kidnapping.
During his hearing, Castro apologised to the three victims who were still teenagers when they were taken captive about a decade ago, and asked for their forgiveness.
"I am truly sorry," Castro had said.
However, he had insisted that he did not beat or torture the three young women.
"I am not a monster. I'm just sick. I have an addiction," said Castro.
Judge Michael Russo, while handing down the sentence, told Castro that he had no place in this world.
"There is no place in this city, there is no place in this country, there is no place in this world for those who enslave others," Russo said.
"You don't deserve to be out in our community. You are too dangerous," added Russo.
He called Castro's behaviour "anti-social personality disorder" and the sentence "commensurate with the harm you've done".
Michelle Knight, 32, was the first of the three women to be kidnapped by Castro and the only victim who appeared in court last month.
"I went through 11 years of hell, and now your hell is just beginning," Knight told Castro.
Amanda Berry, Gina DeJesus and Michelle Knight were rescued May 6 from a house in Cleveland, Ohio.
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