"The sentence reflects the commitment of the international community to do justice to the terrible crimes committed on that day," said UN Spokesperson Stephane Dujarric, in a statement on behalf of Secretary General António Guterres.
However, to date, the condemned Salim Jamil Ayyash remains free.
Trial day
In August, the UN-backed judges of the Special Tribunal for Lebanon found Mr. Ayyash guilty in absentia for the massacre of the prime minister, who was one of the most important politicians in the Sunni-majority Muslim country.
On the morning of the 2005 explosion, Mr. Hariri was traveling in a motorcade past the St George Hotel in downtown Beirut when a bomb that was hidden in a van detonated.
In addition to the death of the former prime minister, the powerful blast killed 22 people and injured 226.
Others acquitted
In 2007, the UN and Lebanon set up the STL to investigate the bombing and treat four suspects in absentia.
Three other defendants - Hassan Habib Merhi, Hassan Hussein Oneissi and Assad Hassan Sabra - were acquitted.
Hezbollah denied any involvement in the bombing and court judges reportedly said there was no evidence to implicate the group's Shiite leaders.
According to media reports, Wissam Eid, a Lebanese investigator who provided mobile phone data that revealed plans to prepare and execute the attack, was killed in Beirut in 2008.
Special Court
The Special Tribunal for Lebanon is an independent tribunal that was established, at the request of the Government, following an agreement between Lebanon and the UN, which was implemented by the Security Council.
"The Secretary General urges the international community to continue supporting the judicial independence that continues in the STL", stressed Mr. Dujarric.
Special Tribunal for Lebanon