The official, who spoke on condition of anonymity because of the sensitive nature of the matter, said Hamdan will serve out the remainder of his sentence in Yemen.
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.
Hamdam's defense lawyers did not immediately respond to messages requesting comment on his release.
Associated Press writer Ben Fox contributed to this report.