The decision to appoint al-Zawahiri leader was made out of respect to the "righteous martyrs" and to honor bin Laden's legacy, the announcement said.
The message also said that the fight against "apostate invaders" will continue "until all invading armies leave the land of Islam."
Al-Qaeda is known to disseminate information through its affiliate websites. Recently, news surfaced that Mi6 agents hacked an Al-Qaeda online magazine, which contained bomb-making instructions, but the network managed to reissue the magazine and later publish four more editions.
Bin Laden, Al-Qaeda's longtime leader, was killed in a secret compound in Abbotabad, Pakistan, in May. Even before the death of bin Laden, al-Zawahiri was considered the leader of Al-Qaeda, but the Thursday's announcement on the jihadist websites makes that official.