The Pentagon said senior US Airman Ahmad al-Halabi of Detroit, Michigan, was in jail at Vandenberg Air Force Base, California, where he was charged with 32 counts, including espionage and other criminal violations.
Defence officials said the 24-year-old al-Halabi was found to have classified information on a computer in violation of strict security rules at the US Naval Base in Guantanamo Bay, Cuba.
Al-Halabi was detained on 23 July at Vandenberg Air Force Base in California as he disembarked a plane returning from Guantanamo.
He has been charged with espionage, aiding the enemy, failing to obey a lawful order, bank fraud, and making a false official statement.
Pentagon officials did not reveal why they had decided to disclose his arrest only two months after it happened.
The penalty for aiding the enemy is "death or such other
punishment as a court-martial or military commission may direct," according to the Uniformed Code of Military Justice.
The action was disclosed just days after the military acknowledged the arrest of a military chaplain at the base, Captain James Yee, a Muslim convert who was taken into custody on 10 September as he got off a military plane at Jacksonville Naval Air Station in Florida.
"At this time, there is no link to Yee. That doesn't mean there won't be," the official said.
No charges have been filed against Yee, who this year
ministered to many of the prisoners at Guantanamo.
The detention camp holds 660 prisoners, most of them taken during the war in Afghanistan, but including men from countries as far apart as Australia and Bosnia.
They are being held there without charge and deprived of legal rights in conditions that have been heavily criticised by human rights organisations.
Many of them are expected to face special military trubunals.