The 41-minute video, posted on an Islamic militant Web site, had footage of al-Zawahri and a man the video identified as Adam Yehiye Gadahn, who has appeared in past al Qaeda videos with the nom de guerre of Azzam al-Amriki.
"To the American people and the people of the West in general ... God sent his Prophet Muhammad with guidance and the religion of truth ... and sent him as a herald," al-Zawahiri said.
Gadahn and al-Zawahiri did not appear together in the footage.
It was the second time Gadahn has appeared in the same video with al-Zawahri. In a July 7 video marking the one-year anniversary of bombings against the London transit system, Gadahn said no Muslim should "shed tears" for Westerners killed by al Qaeda attacks.
"We invite all Americans and unbelievers to Islam," Gadahn said in Saturday's video, dressed in a white robe and turban with a long, thick black beard and a computer terminal in the background.
Gadahn, a 28-year-old American from California who converted to Islam, is wanted by the FBI in connection with possible terrorist threats against the United States.
In May 2004, the U.S. government announced that Gadahn was suspected of attending an al Qaeda training camp and working as a translator for the group. Investigators have said that Gadahn, who grew up on a farm in California, converted to Islam as a teen and moved to Pakistan.
It would be the first time Gadahn has appeared in a video released by al Qaeda. He is believed to benot officially from al Qaeda, given to the American television network ABC in Pakistan in 2004 and a few days before Sept. 11, 2005.
In 2004, Gadahn's father, Philip, said that he was baffled by the allegation his son was an al Qaeda operative, saying he has spoken to his son only occasionally in recent years but still can't imagine that he would be involved with a terrorist network.
Gadahn spoke for much of the video released Saturday, saying he wanted to correct the incorrect image Americans have of Islam.
He described the West as "the civilization which enslaved Africa, slaughtered native Americans, fired bombs at ... Tokyo and (the Iraqi city of) Fallujah and nuked Hiroshima and Nagasaki."
He said it shows more concern for archaeological sites, like statues of Bhudda destroyed by Afghanistan's former Taliban rulers, "than it shows of the people of Afghanistan and Iraq."
He said "ignorance" of Islam "causes the people of the West to rapturously applaud when Israel perpetrates wholesale slaughter of Muslims in Lebanon and Palestine and leads them to give their consent to the atrocities that governments commit in Afghanistan, Iraq and elsewhere in the Muslim world."
The video, issued by al Qaeda's production wing As-Sahab, had been advertised on militant Web sites for several days before it appeared Saturday.
Besides the July 7 video, Gadahn is believed to be a masked figure who appeared in two previous videos not officially from al Qaeda, given to the American television network ABC in Pakistan in 2004 and a few days before Sept. 11, 2005.
In the 2005 tape, the speaker threatened new terror attacks in Los Angeles and Melbourne, Australia. The 2004 tape praised the Sept. 11 attacks and said a new wave of attacks could come at any moment.