The Egyptian group, Gamaa Islamiya, is apparently a revived version of a 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-Zawahiri said in a videotape aired on Aljazeera on Saturday.
Al-Zawahiri said the Egyptian group was led by Muhammad al-Islambouli, the younger brother of Khaled al-Islambouli, who assassinated Anwar al-Sadat, the then Egyptian president, in 1979 and was later executed.
Muhammad 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 armed 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.
The Egyptian government detained many thousands of Gamaa Islamiya members or sympathisers in the 1990s, when the group was waging a low-level guerrilla war against the security forces, mainly in the south of the country.
Egypt has seen a string of attacks
on tourists since October 2004
Hundreds have come out of detention over the years after renouncing the use of violence to overthrow the government.
And its leaders declared a truce with the government in 1997, after an attack on tourists in Luxor.
The group has not claimed any attacks since the late 1990s.
Egypt has seen a string of 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 fighters.