Speaking to the BBC's World Today Programme, Mark Denbeaux, who represents some of the foreign detainees, said the man was among 141 prisoners scheduled for release.
Mr Denbeaux said US officials had not yet told the prisoner, who the Pentagon has named as Mani Shaman Turki al Habardi Al-Utaybi, of his impending release.
He went on to add that the reason behind this was because US authorities had not yet decided on a repatriation destination.
“However our policy was we would refuse to tell people who were scheduled to be released until we had a location,” Mr Denbeaux said.
“So we had decided this was a safe person, free to be released but we needed a country to send him to, and his despair was great enough and in his ignorance he went and killed himself.”
A law professor who is representing two Tunisians at Guantanamo, Mr Denbeaux, said earlier that a "stench of despair” hung over the US prison camp and that detainees were shutting down and quitting".
“These people are told they’ll be 50 by the time they get out, that they have no hope of getting out. They’ve been denied a hearing, they have no chance to be released,” he told the BBC.
New calls to close Guantanamo
The other two men who died on Saturday morning were named as Ali Abdullah Ahmed and Yassar Talal al-Zahrani.
Ahmed was from Yemen, while the other two were from Saudi Arabia.
The three suicides have intensified calls for the war-on-terror prison camp to be closed while a US diplomat has called the hangings a "good PR move" to gain attention.
Defence lawyers and critics say depression is gripping inmates, including Australian David Hicks who is being held in a concrete solitary confinement cell.
However, the Pentagon's Southern Command chief suggested yesterday's deaths might have been aimed at swaying the US Supreme Court as it decides the legality of military tribunals for Hicks and nine others ordered by President George W Bush.
"This may be an attempt to influence the judicial proceedings in that perspective," said General Bantz Craddock.
Rear Admiral Harry Harris, the camp's commander, described the suicides as an act of "warfare".
"These are dangerous men and they will do anything they can to gain support for their cause and the advance of their cause," Mr Harris said.
Colleen Graffy, US deputy assistant secretary of state for public diplomacy, told the BBC the suicides were a "good PR move to draw attention".
"It does sound that this is part of a strategy in that they don't value their own life and they certainly don't value ours and they use suicide bombings as a tactic to further their Jihadi cause," she said.
Ms Graffy coordinates efforts with Karen Hughes, a former top aide to President George W Bush who is now a special envoy charged with trying to improve the US image abroad, especially in Islamic countries.
Hicks “desperate for contact”
The only Australian held at Guantanamo Bay, David Hicks, was described by his lawyer as desperate for human contact.
Major Michael Mori said David Hicks, who has been couped up in the Cuban detention centre for more than four years, was in poor health, suffering weight loss and continuing signs of depression.
"I found him very desperate for human contact," said Mr Mori, who visited Mr Hicks last week.
"You could just tell when I first got to see him he was just so hungry to interact with another human being," he told Australian Broadcasting Corp. radio.
Prime Minister John Howard, who says Mr Hicks must face US justice and has refused to work for his release, said the detainee had received a consular visit two weeks ago, which raised no concerns.
"I have been told that he received a consular visit about two weeks ago and the report from that consular visit was positive," Mr Howard, a steadfast ally of US President George W. Bush, told reporters.
But Mr Bush said he would like to empty Guantanamo and is working to repatriate many of the detainees.