Guards administered first aid to the prisoner before he was rushed to a base hospital, where he was declared dead "after extensive lifesaving measures had been performed," the U.S. military's Southern Command said in a brief statement.
The prisoner's name and nationality were not released. But U.S. officials, speaking on condition of anonymity because they were not authorized to release his identity, said he was from Yemen.
Experts have yet to determine what caused the early-afternoon death since there were no obvious signs, said Navy Capt. Robert Durand, a spokesman for the prison.
"We haven't ruled in or out anything," Durand said. "There is no apparent cause, natural or self-inflicted."
The prisoner was the ninth detainee to die at the facility since it was opened in January 2002 to hold men suspected of terrorism or links to al-Qaida and the Taliban. The military has said two of the previous deaths were by natural causes and six were declared suicides.
Former prisoner Moazzam Begg, now director of the advocacy group Cageprisoners, said the death underscores the bleak outlook for the 167 men held at Guantanamo.
"Almost 11 years since the camp opened few people are concerned whether the men
The latest death occurred in Camp 5, a section of the prison used mostly to hold prisoners who have broken detention center rules.
This prisoner had recently splashed a guard with what military officials call a "cocktail," typically a mixture of food and bodily fluids, which is why he was on disciplinary status, Durand said.
He was on a hunger strike earlier this year but stopped it on June 1 and was at 95 percent of his ideal body weight and 14 pounds heavier than when he came to Guantanamo, the spokesman said.
Prisoners at Guantanamo include a handful charged with war crimes, including five accused of helping carry out the Sept. 11 terror attack on the U.S. More than half the population has been cleared for release, but the government says it is has been unable to find stable countries to transfer them to.
Military officials have significantly improved conditions for detainees who don't break prison rules, such as providing them with classes and satellite television in a communal section known as Camp 6. But conditions are stark in Camp 5 and a nearby disciplinary annex known as Camp 5 Echo. Both are akin to a maximum-security U.S. prison, the men spending much of each day enclosed in solid-walled cells.
Wells Dixon, a lawyer who has represented a number of Guantanamo prisoners, said the sense of despair among prisoners overall seems to have worsened since the Supreme Court announced in June that it would not review the way courts were handling the men's individual challenges to their confinement.
"The mood is very dark," Dixon said. "There are a lot of guys who are having a really hard time ... Many of them have lost any hope that they are ever going to be released regardless of their status."
Durand, the prison spokesman, said the man who died Saturday had not been charged and was not designated for prosecution.
A medical examiner was sent to the base to determine the exact cause of death, and an investigation will be conducted by the Naval Criminal Investigative Service, which is standard in the death of detainees at Guantanamo. Southern Command will also review the incident to determine how it occurred and whether any internal policies need to be addressed.
Durand said the U.S. government was working to notify the man's family and his country before releasing his name and nationality.
"Certainly we don't want the family finding out in the media before they have been notified," he said.
A mortuary team washed and placed the body in a shroud in accordance with Islamic burial rites for shipment to the prisoner's homeland.
The most recent death was in April 2011, when a 37-year-old Afghan prison died in an apparent suicide. His lawyer told The Associated Press at the time that the man had a long-term mental illness and had tried to kill himself at least once before. Two Saudis and a Yemeni prisoner were found hanging in their cells in June 2006 in what the military determined was a coordinated suicide.
Associated Press writer Lolita Baldor in Washington contributed to this report.