The White House says a US attack on al-Qaeda in the border region of Pakistan and Afghanistan killed the 36-year-old, who frequently appeared in al-Qaeda videos.
He was known to the US for some years. In 2006, Gadahn became the first US citizen to be charged with treason since World War II.
The indictment said he had "knowingly adhered to an enemy of the United States... with intent to betray the United States". A $1m bounty was placed on his head.
He was home schooled and raised as a Christian in California before converting to Islam at 17.
Following his conversion, he moved in 1998 to Pakistan and married an Afghan refugee.
Gadahn performed translations for al-Qaeda and become associated with al-Qaeda's captured field commander, Abu Zubaydah. He is also thought to have later trained at a militant camp in Afghanistan.
In 2004, the US justice department named him as one of seven al-Qaeda operatives planning imminent attacks on the US. Shortly afterwards, he appeared in a video on behalf of al-Qaeda, identifying himself as "Azzam the American".
In September 2006, he appeared in a video with Ayman al-Zawahiri and exhorted his fellow Americans to convert to Islam and support al-Qaeda.
In 2011, he appeared in a video exulting in the new information revealed about US global interests through the Wikileaks publication of classified documents, according to prosecutors at the trial of Pte Bradley Manning.
He has also appeared in multiple other videos urging Muslims in the West to carry out attacks.
Analysts say Gadahn is not part of al-Qaeda's senior leadership, and does not hold any operational or ideological significance.
Before his apparent death at the hands of the US military in a probable drone strike, Adam Gadahn had risen to be one of the US government's most wanted men as he became a high profile al-Qaeda propagandist.