The Special Tribunal for Lebanon said Salim Ayyash was guilty as a co-conspirator of five charges linked to his involvement in the suicide truck bombing.
Mr Hariri and 21 others were killed and 226 were wounded in a huge blast outside a seaside hotel in Beirut on February 14, 2005.
After a years-long investigation and trial, three other Hezbollah members were acquitted of all charges that they also were involved in the killing of Mr Hariri, which sent shock waves through the Middle East.
None of the suspects were ever arrested and were not in court to hear the verdicts.
The tribunal’s judges also said there was no evidence the leadership of the Hezbollah militant group and Syria were involved in the attack, despite saying the assassination happened as Mr Hariri and his political allies were discussing calling for an “immediate and total withdrawal of Syrian forces from Lebanon,” Presiding Judge David Re said.
The verdicts came at a particularly sensitive time for Lebanon, following the devastating explosion at the Port of Beirut two weeks ago, and as many in Lebanon are calling for an international investigation into that explosion.
But it was doubtful the verdict, coming 15 years after the assassination and with no defendants in court, would bring closure to those who had been waiting for justice.
Mr Hariri’s son Saad, himself a former Lebanese premier, told journalists outside the court building that the family accepts the verdicts, though he acknowledged that “everybody’s expectation was much higher than what came out”.
“The time when political crimes in Lebanon used to go unpunished are gone,” he said, adding that the family awaits the “implementation of justice”.
A hearing will be held at a later date to determine Ayyash’s sentence. As the UN-backed court has no death sentence, the maximum sentence is life imprisonment.
Sketching the complex political backdrop for the assassination, the judge said that in the months before his death, Mr Hariri was a supporter of reducing the influence of Syria and Hezbollah in Lebanon.
He said judges who studied reams of evidence in the trial of the four Hezbollah members accused of involvement in the bombing were “of the view that Syria and Hezbollah may have had motives to eliminate Mr Hariri, and some of his political allies”.
But he added that there was no evidence the “Hezbollah leadership had any involvement in Mr Hariri’s murder, and there is no direct evidence of Syrian involvement in it”.
The scale of the investigation and trial was apparent from the size of the written judgment. The judge said it ran to more than 2,600 pages with some 13,000 footnotes.
Prosecutors based their case largely on data from mobile phones allegedly used by the plotters to plan and execute the bombing.
Judge Re said that the telecom evidence in the case was “almost entirely circumstantial”.
However, another judge, Janet Nosworthy, later said that judges had ruled that four different networks of mobile phones “were interconnected and coordinated with each other, and operated as covert networks at the relevant times”.
It was the lack of clear evidence from phone records tying them to the bombing plot and efforts to set up a false claim of responsibility that led to the acquittals of three suspects — Assad Sabra, Hassan Oneissi, who changed his name to Hassan Issa, and Hassan Habib Merhi.
The judges ordered the arrest warrants for the three men to be withdrawn.
During the trial, which started in 2014 and spanned 415 days of hearings, the tribunal in Leidschendam, Netherlands, near The Hague, heard evidence from 297 witnesses.
Initially, five suspects were tried, all of them Hezbollah members. Charges against one of the group’s top military commanders, Mustafa Badreddine, were dropped after he was killed in Syria in 2016.
The court said on Tuesday it could not prove that Badreddine was the mastermind behind the assassination.
Ayyash, 56, has been at large and is not likely to serve time as Hezbollah had vowed not to hand over any suspects. Prosecutors and defence lawyers can appeal against the verdicts.
