Salim Ayyash, 57, was found guilty in absentia of murder and terrorism on August 18 by the Netherlands-based Special Tribunal for Lebanon over the huge suicide bombing that killed the Sunni billionaire politician and injured 226 others.
Ayyash remains on the run, with Hassan Nasrallah, the head of the Shiite Hezbollah movement, refusing to hand him over or to recognise the UN-backed court.
"Mr Ayyash participated in an act of terrorism that caused mass murder," chief Judge David Re said as he announced the sentence.
"The trial chamber is satisfied that it should impose the maximum sentence for each of the five crimes of life imprisonment, to be served concurrently."
The court has issued an international arrest warrant for Ayyash for the "extremely grave" crimes.
The Hezbollah chief's statements on refusing to surrender Ayyash led Judge Re to make the "inference" that he was being shielded from justice, he said.
'Inflicted terror'
After a years-long trial, Ayyash was convicted in August of conspiracy to commit a terrorist act, committing a terrorist act using an explosive device, the "intentional homicide" of Hariri, and of 21 other people, and attempted homicide of those injured in the attack.
The court said there was not enough evidence to convict three other defendants who were also on trial in absentia -- Assad Sabra, Hussein Oneissi and Hassan Habib Merhi.
The attack on Hariri, who served as Lebanon's prime minister until he resigned in October 2004, changed the face of the Middle East.
He was killed in February 2005 when a suicide bomber detonated a van filled with explosives as his armoured convoy drove past on the Beirut seafront.
The "massive" bombing left 11-metre (33-foot) wide crater was "intended to and in fact inflicted terror", the court said.
The attack triggered mass protests that drove Syrian forces out of Lebanon after three decades. Hariri had opposed the Syrian presence in Lebanon.
Judges said Ayyash was at the centre of a network of mobile phone users who scoped out Hariri's movements for months before his assassination.
Sentencing Ayyash on Friday, the court said while there was no direct evidence of Syria or its ally Hezbollah's involvement, the attack "most probably had to involve state actors".
"The state with the most to gain from Mr Hariri's assassination was probably Syria," judge Janet Nosworthy said.
'Culture of impunity'
Prosecutors had said five concurrent life terms were the "only just and proportionate sentence" for Ayyash, given it was the "most serious terrorist attack that has occurred on Lebanese soil."
Legal experts said the sentencing was still important, even without Ayyash in the dock.
"In absentia trials are of course not the ideal way of dispensing international justice," Christophe Paulussen, senior researcher at the Asser Institute in The Hague, told AFP.
International tribunals were like "a giant without arms and legs" since they relied on states to arrest suspects and could not enforce orders themselves.
"But even with this handicap, the STL has now at least established a very authoritative judicial record about what happened 15 years ago, thus assisting the Lebanese society in moving away from a culture of impunity towards one of accountability," said Paulussen.
The UN Security Council agreed in 2007 to establish the court, billed as the world's first international tribunal set up to probe terrorist crimes.
It opened its doors in 2009, although the Hariri trial itself did not formally start until 2014.
The court has cost at least $600 million to operate and has so far heard only four cases, two of them for contempt of court about news reports with information about confidential witnesses.
Ayyash faces a separate case at the tribunal over three other deadly attacks on Lebanese politicians in 2004 and 2005.
