Department of Corrections spokeswoman Gretl Plessinger says Couey of natural causes Wednesday at a Jacksonville hospital. The 51-year-old had been.
"It was not a surprise," she said.
Plessinger declined to provide any specifics, citing a federal law protecting the privacy of hospital patients.
Jessica's grandmother, Ruth Lunsford, 77, said in a telephone interview that she couldn't feel bad about Couey's death.
"He murdered my granddaughter. He didn't show any mercy to my granddaughter," she said. "God took control of it. He took him out of this world. ... I'm not crying, honey. If my legs and feet would hold up, I'd go out and shout all over Citrus County."
The crime prompted many states to pass laws named for Jessica that impose restrictions on sex offenders, including tougher penalties and registration requirements. Florida's version also bans them and others convicted of serious crimes from school grounds.
Couey died just over a month before the Florida Supreme Court was scheduled to hear his automatic appeal. He had an IQ of 78, slightly above the level generally considered mentally disabled, but the judge rejected an argument by his lawyer that he couldn't legally be executed.
The jury that convicted Couey in March recommended 10-2 that he die for his crimes, though the ultimate decision was left to the judge.
Couey, already a convicted sex offender when he committed the crime, was arrested in Georgia and confessed to the killing. That confession was thrown out as evidence because Couey did not have a lawyer present.
Couey spent much of his 2007 trial, which was moved to Miami because of publicity about the case, drawing in coloring books. He looked straight ahead as Circuit Judge Ric Howard told him he should be executed.
Jessica's father, Mark Lunsford, teared up then as he listened to the judge read a detailed history of the case. Outside court, he had a message for Couey: "Skip all these appeals. Take your punishment. Stand up and be a man."
Couey took Jessica from her bedroom to his nearby trailer in February 2005, triggering a massive search. The third-grader's body was found about three weeks later in a grave in Couey's yard, only about 150 yards from her home.
Couey's confession was thrown out as evidence because he did not have a lawyer present. Jail guards and investigators testified he repeatedly admitted details of the slaying but said he hadn't meant to kill the girl.
Prosecutors also introduced overwhelming physical evidence, including fingerprints and DNA.
Jessica's body was found wrapped in two garbage bags under a foot of dirt.
Couey previously had been convicted of exposing himself to a 5-year-old girl in 1991. His criminal record also included 24 burglary arrests and carrying a concealed weapon.