"We announce that al-Tawhid wa al-Jihad, its leader and soldiers have pledged allegiance to Usama bin Ladin," read the statement, the first linking al-Zarqawi to al-Qaida.
However, its authenticity could not be verified.
Both al-Zarqawi and bin Ladin have a $25 million US bounty on their heads.
Since the US invaded Iraq in April 2003, Washington has held al-Zarqawi responsible for attacks against its forces and US-sponsored officials.
The internet statement, however, says: "Shaikh Abu Musab al-Zarqawi was in contact with the leadership of al-Qaida for eight months. They exchanged points of view and then there was a cut due to fate.
"After contact was restored, the al-Qaida leadership understood our strategy in Iraq," the statement added.
Al-Zarqawi just joined
London-based Muslim activist Yasir al-Sirri, whose own website monitors other Muslim sites, said he believed the statement was authentic and was aimed at unifying ranks among anti-US groups.
"It also aims to belie US claims that al-Zarqawi was an al-Qaida member by saying he only just joined the network"
London-based Muslim activist
"It is aimed at boosting morale and recruiting more people by saying that Tawhid and Jihad have become the soldiers of bin Ladin in Iraq," he said.
"It also aims to belie US claims that al-Zarqawi was an al-Qaida member by saying he only just joined the network," he added.
Al-Zarqawi also faces charges in his native Jordan after being indicted on Sunday, along with 12 other people for allegedly plotting to bomb US and government buildings.
The charge sheet lists the US embassy in Amman and the Jordanian intelligence headquarters as prime targets.
If found guilty, all face the death penalty.