The 46-year-old was nominated Thursday to form a cabinet by his one-time political adversary, President Michel Aoun, who took office this week after receiving the surprise support of his old foe.
Hariri, who has already served as prime minister once before, has a political career marked by his opposition to the powerful Shiite movement Hezbollah, which is allied with Aoun.
The movement is a key backer of the government in neighbouring Syria, which Hariri accuses of having planned his father's murder.
He was a leading proponent of the departure of Syrian forces from Lebanon in 2005, after mass demonstrations following the assassination.
Hariri, who now sports a beard along with his trademark slicked-back locks, returns to the office in a bid to restore the standing of Lebanon's Sunni community and counterbalance Hezbollah's influence.
Born in Saudi Arabia, where his father made his fortune, he was running the family's Oger construction firm when Rafik Hariri was assassinated in February 2005.
At his family's urging, he returned to Lebanon to enter politics, heading an anti-Syrian bloc to victory in the 2005 legislative elections.
Confrontations with Hezbollah
In August 2007, he formed the Future Movement party, a majority-Sunni bloc, which came out ahead in the 2009 legislative elections, winning 33 of the parliament's 128 seats.
In November that year, he became prime minister for the first time, forming a unity government with Hezbollah and its allies after marathon negotiations.
But the government only lasted until January 2011, when Hezbollah and its allies pulled their ministers from the cabinet, forcing its collapse.
Tensions had already nearly boiled over in May 2008, when Hezbollah fighters seized parts of Beirut after pitched battles with Future Movement supporters.
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The crisis raised fears of a new conflict in the country, still scarred by its 1975-1990 civil war.
Hariri was also locked in a standoff with Hezbollah over funding for the Special Tribunal for Lebanon, which is prosecuting his father's murder.
The tribunal has implicated Hezbollah members in the assassination, but the group dismisses the body as a US-Israeli conspiracy.
Hariri's differences with Hezbollah have only deepened with the war in neighbouring Syria, where the powerful Shiite group has dispatched fighters to bolster President Bashar al-Assad's government.
Hariri by contrast has backed the uprising against Assad, and led the calls for Syria to withdraw its forces from Lebanon -- 30 years after their arrival -- in 2005.
Hezbollah is backed by Iran while Hariri enjoys the support of Tehran's regional rival Saudi Arabia.
Dwindling fortune, influence?
Hariri has Saudi citizenship and has tirelessly praised the kingdom, to which he returned after the collapse of his government, citing security concerns.
His wife Lama Bashir-Azm, who is of Syrian origin, and their three children have stayed in Saudi Arabia, even as Hariri began spending time in Lebanon again from 2014.
In June 2016, he announced his permanent return to Lebanon, though he continues to spend periods in Saudi Arabia, where the Hariri business empire has struggled of late.
Hariri's influence with the Saudi royal family also appears to have dwindled since the death of King Abdullah, and in Lebanon he has faced criticism within his Sunni constituency for his lengthy absence and failure to bolster the community.
Former justice minister Ashraf Rifi launched a major challenge to his position as presumptive leader of Lebanon's Sunnis in June 2016, running a rival list in municipal elections in the Sunni stronghold of Tripoli.
A business graduate from Georgetown University in Washington DC, Hariri was virtually unknown before his arrival on the political scene after his father's death.
A polyglot, he was nonetheless mocked for his poor public speaking skills, and initially derided as a political naif.
But his decision to back former rival Aoun for the presidency, ending a vacuum of more than two years, illustrated his comfort with the shifting sands of Lebanon's treacherous political landscape.