Hamdan was convicted of aiding al-Qaida in August and sentenced to 5 1/2 years in prison. He would be eligible for release in January with credit for time served.
The official, who spoke on condition of anonymity because of the sensitive nature of the matter, said Monday that Hamdan will serve out the remainder of his sentence in Yemen.
Waleed Alshahari, who ovesees Guantanamo Bay issues for the Yemen Embassy in Washington, said he was surprised to learn plans for Hamdan's release because there have been no new negotiation on the release of the 90 or so Yemeni detainees at the prison.
He said any deal over their release likely will come under President-elect Barack Obama's administration.
"It seems the new administration wants to close this prison, so so there will be negotiations with them," he said. Security has been a roadblock. The U.S. is concerned the detainees will be released as soon as they are returned to Yemen. Yemeni and U.S. officials agree there should be a new, secure rehabilitation center built in Yemen but officials there say they can't afford it and have asked the U.S. to build it.
Alshahari said he believes the Obama administration will seek a deal with a neighboring country to help pay for the project.
Charles Swift, one of Hamdan's defense lawyers, told The Washington Post: "Certainly the fair thing to do is to return him. If you don't, you really come to the absolute thing of the commissions becoming a complete sham."
A jury of six U.S. military officers sentenced Hamdan at Guantanamo's first war-crimes trial earlier this year, and at the time he had already served five years and a month at the Cuba facility.
Pentagon officials had suggested all along that they could hold the 40-year-old Guantanamo prisoner indefinitely regardless of the sentence. The Pentagon reserves the right to hold him and other "enemy combatants" who are considered dangerous to the United States, even those who are acquitted or complete sentences in the tribunal system.
Guantanamo prosecutors had sought a sentence of 30 years to life for Hamdan, whose trial inaugurated the special commission system in July. They also had argued that as an "enemy combatant" he should not receive credit for his time detained there. A military judge rejected that argument.
While convicted of supporting terrorism, Hamdan was acquitted by a jury of military officers of providing missiles to al-Qaida and knowing his work would be used for terrorism. He was cleared of being part of al-Qaida's conspiracy to attack the United States.
He could have faced up to life in prison and his relatively light sentence was considered a rebuke to military prosecutors who portrayed him as a hardened al-Qaida warrior.
