A former minister of economy, Flaihan had been flown to France with severe burns days after the bombing that killed a total of 21 people, now including Flaihan, and wounded more than 100 others.
Flaihan died on Monday morning in hospital "after vital organs of his body stopped functioning," Future Television reported. The TV station is owned by the al-Hariri family.
Flaihan was sitting in the front seat of al-Hariri's car when the bomb exploded as the former prime minister's motorcade was passing along a seafront boulevard in central Beirut.
Flaihan, who had an MA from Yale university and a PhD in economics from Columbia in the United States, worked for the World Bank and the UN Development Programme before starting his political career in 2000 when he became an MP.
Fleihan is survived by his wife Yasma and two young children.
The assassination of al-Hariri, who had resigned as prime minister in October, triggered a political crisis in Lebanon that is still going on. Opposition leaders accused Syria and Lebanon's pro-Syrian government of playing a role in the killing - a charge both authorities denied.
Mass protests caused the Lebanese government to fall two weeks after the bombing, and the country has still not managed to form a new cabinet. The second legislator to be nominated as prime minister-designate, Najib Miqati, began consulting legislators on Monday on forming a government.
The bombing also greatly intensified international pressure on Syria to withdraw its 14,000 troops from Lebanon. Syria began withdrawing them in March and has pledged to withdraw all its troops and intelligence officers by the end of April.