"Coming soon by the will of God, an invitation to reflection and repentance," read a banner produced by al Qaeda's media wing, al-Sahab, and displayed on a militant Web site.
An image of the bearded Gadahn wearing a red-and-white checkered Arab scarf appears on the banner.
The California-born Gadahn, also known as Azzam al-Amriki, was charged with treason in the U.S. in 2006 and has been wanted since 2004 by the FBI, which is offering a $1 million reward for information leading to his arrest or conviction.
He last appeared on a video in August threatening new attacks on foreign embassies.
"The FBI and our intelligence community partners will review the latest tape for any intelligence value. The hunt for Adam Gadahn continues and we won't ever give up," FBI spokesman Richard Kolko said. "We are employing the full force of the FBI to find and arrest him. The constant pressure greatly decreases his chance of successfully conducting any terrorist operations."