The 28-minute video, "The Noble Knight Dismounted," showed Zawahiri describing bin Laden as "the noble emir" and a "heroic fighter," The New York Times reported, quoting the SITE Intelligence Group, a jihadist communications tracking group that translated the video.
Zawahiri was quoted as vowing to carry on bin Laden's efforts to "expel the invaders from the land of Islam."
The Washington Post quoted U.S. intelligence analysts as saying the Zawahiri tape showed al-Qaida had not yet found a successor to bin Laden and noted Zawahiri made no claim in the tape that he would take control of the terrorist network he helped found.
"This suggests there are still disagreements within al-Qaida about who its next leader should be," a U.S. official told the Post.
There have been reports Saif al-Adel, an Egyptian like Zawahiri, had been picked to succeed bin Laden, but U.S. officials doubt the claim, the Post said.
Bin Laden was killed May 2 by a U.S. Navy SEAL team during a raid on his compound in Pakistan's garrison town of Abbottabad.
In the tape, Zawahiri warned of reprisals against the United States.
The Zawahiri tape urged Pakistanis to revolt against their leaders, "just as your brothers in Tunisia, Egypt, Libya and Syria have done," the Times reported.
The report said Zawahiri is believed to be hiding in Pakistan.