"The number of victims from the attack rose to seven after the death of young Mohammad al-Chaar this morning, succumbing to his injuries at the American Hospital in Beirut," said Lebanon's official NNA news agency.
Following last day's bombing in the heart of Beirut, AFP photographers saw the 16-year-old lying on the pavement, his head bloodied, before he was taken to hospital with severe injuries.
Chaar had been identified online as one of four youths seen posing in a group selfie posted on social media networks in front of the car that exploded moments later.
Mohammad Chatah, an influential economist, former finance minister and ex-envoy to Washington who was also a close aide to ex-prime minister Saad Hariri, was killed in the attack.
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Chatah was also a leading critic of Syrian President Bashar al-Assad, and the opposition Syrian National Council accused Damascus and its allies Iran and Hezbollah of being behind the attack.
Dozens of people were also wounded by the bomb, which officials said contained 50-60 kilos of explosives.
Saad's father Rafiq Hariri, also a former prime minister, was killed in a suicide bombing on the Beirut seafront on February 14, 2005.