The Egyptian group, Gamaa Islamiya, is apparently a revived version of a militant group that waged a campaign of violence in Egypt during the 1990s but was crushed in a government crackdown.
"We announce to the Islamic nation the good news of the unification of a great faction of the knights of the Gamaa Islamiya ... with the Al Qaeda group," Ayman al-Zawahri, the deputy leader of al Qaeda said in the videotape aired on the Al-Jazeera news network.
Al-Zawahri said the Egyptian group was led by Mohammed al-Islambouli, the younger brother of Khaled al-Islambouli, the militant who assassinated Egyptian President Anwar al-Sadat in 1979 and was later executed.
Mohammed al-Islambouli left Egypt in the mid-1980s and was believed to have been in Afghanistan working with al Qaeda-leader Osama bin Laden, said Diaa Rashwan, an Egyptian expert on militant groups.
It was the first time that al Qaeda has announced an Egyptian branch, but it was not clear whether the new version of Gamaa Islamiya really has a presence on the ground in the country. Its previous incarnation was largely eliminated by the government crackdown, and its leaders later announced a truce from prison. It has not claimed any attacks since the late 1990s.
Rashwan said al-Zawahri's claim was likely just propoganda.
"This is media talk from Ayman al-Zawahri. The Gamaa Islamiya has its own leadership and they said they have already rejected joining al Qaeda in the past," he said. "Gamaa Islamiya has no command outside Egypt. They have dissolved in Egypt."
Egypt has seen a string of terror bombings against tourist resorts in the Sinai Peninsula since October 2004, killing 98 people. Egyptian authorities have said those attacks were carried out by a group calling itself Monotheism and Jihad, with links to Palestinian militants.
Many experts believe Monotheism and Jihad is at least inspired by al Qaeda and may have some operational links, but the Egyptian government has not announced any connection.