"The shura (consultative) council and al-Qaeda in the Land of Two Rivers have both agreed to appoint Sheikh Abu Hamza al-Muhajir to succeed Abu Musab al-Zarqawi in the leadership of the organisation," said a statement on Al-Hesba website, where Al-Qaeda's official statements usually appear.
Its authenticity could not be independently verified.
"Sheikh Abu Hamza is a pious brother with a jihadi track record and a solid scholarly background. We ask God to bless him and to help him finish what Sheikh Abu Musab has started," the statement added.
His nationality was not given but the word "muhajir" means immigrant in Arabic, indicating the incoming leader is not from Iraq.
Muhajir, little known in the West, was not among the names that al-Qaeda experts had expected to succeed Zarqawi.
US President George W Bush shortly afterwards said the successor "is going to be on our list to bring to justice".
Al-Qaeda makes up about five percent of Iraq's Sunni Arab insurgency but its suicide bombers have been responsible for the most spectacular violence, sometimes killing over 100 people in a single attack.
Although US and Iraqi leaders have hailed Zarqawi's death in an American air strike as a major blow against al-Qaeda, it is widely accepted that it will not halt the violence in Iraq.
The group on Sunday released a statement vowing to avenge Zarqawi's death with "great operations to shake the enemy."
The Consultative Council of the Mujahedeen, which is an alliance of eight militant Sunni Arab groups formed last January and led by Al-Qaeda in Iraq, also renewed its allegiance in that statement to Al-Qaeda supremo Saudi-born fugitive Osama bin Laden.
Lung damage
Meanwhile, the US military said autopsy results showed that Zarqawi died of serious damage to the lungs caused by the shockwaves from a US air strike on his hideout, which involved two 500-pound bombs.
Dismissing claims the militant was beaten to death after coalition forces found him alive, US military doctor Colonel Steve Jones told reporters in Baghdad that blast waves from the bombs caused tearing, bruising of the lungs and bleeding.
"This wound was not immediately fatal. Death occurred as lung function deteriorated and the lungs became progressively unable to absorb oxygen into the bloodstream."
Baghdad bombs
There have been two separate car bomb attacks in Baghdad's Sadr City and western Baghdad, killing at least 10 people and wounding 51.
And the US military said it killed nine people in an attack on a terrorist cell, however witnesses said the dead are civilians and include two children.