The name was not immediately known. The name al-Muhajer, Arabic for "immigrant," suggested he was not Iraqi.
"Al Qaeda in Iraq's council has agreed on Sheik Abu Hamza al-Muhajer to be the successor for Abu Musab al-Zarqawi in the leadership of the organization," said a statement signed by the group on an Islamic militant Web forum where it often posts messages.
It said al-Muhajer was "a beloved brother with jihadi (holy war) experience and a strong footing in knowledge."
The authenticity of the statement could not be independently confirmed. Zarqawi, who founded the group, was killed Wednesday in a U.S. airstrike northeast of Baghdad.