Castro, who was serving a life sentence plus 1,000 years, was being held at the Correctional Reception Center in Orient, Ohio. A Corrections Department spokeswoman told the Chicago Tribune that he was under protection and isolated from other inmates.
Prison staff found the 53-year-old hanging in his cell as they made their nightly rounds. When medical staff couldn't resuscitate him, Castro was transferred to the hospital where he was pronounced dead.
His attorney, Jaye Schlachet, told Reuters that the former bus driver was repeatedly denied access to a psychologist.
"We requested the opportunity for our retained independent psychologist to see and evaluate Mr. Castro in both the county jail and in the prison reception center, where he was being held. We were denied and thwarted in each of our attempts by the state and county," he said.
The apparent suicide could be difficult on victims Amanda Berry, Gina DeJesus and Michelle Knight. Dr. Linda Papadopoulos told NBC News, "He decided his fate, something they were never ever able to do for themselves. He had ultimate control. To some extent this was in a way his last slap to their faces -- 'I've got this over you'."
Castro abducted the three women in Cleveland and held them captive in his nondescript home. Berry made national headlines last May after her dramatic escape.