Detlev Mehlis, the chief prosecutor in the office of the attorney general in Berlin, was picked on Friday by UN Secretary-General Kofi Annan because of his experience in investigating terrorist and complex transnational crimes including the 1986 bombing of a Berlin discotheque frequented by US soldiers, UN diplomats said.
"Mr Mehlis will travel to Beirut as soon as possible to begin the commission investigation," UN spokeswoman Marie Okabe said.
The UN Security Council in April ordered an outside inquiry into Hariri's murder after a UN fact-finding mission, led by Irish Deputy Police Commissioner Peter Fitzgerald, concluded that Lebanon's inquiry into the killing had "serious flaws" and could not reach a credible conclusion.
About 50 people, including administrative and security staff, are expected to participate in the investigation. Hariri was killed by a car bomb on 14 February in Beirut.
Mehlis has 25 years of experience as a prosecutor, including stints as the individual responsible for terrorism and organized crime cases, and has led investigations into "serious, complex transnational crimes," Okabe said.
Hariri's death brought dramatic
political change in Lebanon
In the discotheque case, a Berlin court ruled in 2001 that the Libyan secret service was behind the bombing at the West Berlin "La Belle" nightclub, in which a Turkish woman and two US soldiers were killed and more than 200 people injured.
The attack on Hariri took place after he accused Syria of meddling in Lebanon's internal politics. Mass street demonstrations followed, and Lebanese opposition politicians blamed Damascus for his death.
Syria, which dominated Lebanon for three decades, denied any involvement in the killing but agreed to withdraw its troops from Lebanon under international pressure.
A UN team sent to the region to verify the Syrian withdrawal was headed back to New York and would report its findings to Annan early next week, Annan said on Friday.