Adam Gadahn, 28, also has been charged with aiding terrorists for appearing in al-Qaida videos, reported the Associated Press.
The crime of treason, as outlined in the Constitution, requires two witnesses or an in-court confession for a conviction.
Gadahn, a California native, is believed to have participated in al-Qaida training camps in Pakistan and acted as a translator.
He has appeared in a video with al-Qaida's No. 2 leader, Ayman al-Zawahiri, and made other recordings where he called for the world to convert to Islam and hailed the Sept. 11, 2001, hijackers, according to CNN.
Gadahn is believed to be overseas, not in U.S. custody.
He grew up on a goat farm in Riverside County, California, and left home as a teenager, his father said.
He converted to Islam and worshipped at the Islamic Society of Orange County in 1997 before being expelled for attacking one of its leaders, reported the AP.
His mother last spoke to him by phone in March 2001 while he was in Pakistan, working at a newspaper.
Holder: 9/11 Trials Will Weigh 'Crime of the Century' 
