A new government will also have to contend with a COVID-19 surge and the fallout of the August explosion at Beirut port that killed nearly 200 people and caused billions of dollars of damage.
The blast prompted the outgoing government, which succeeded Hariri's last administration, to quit.
At 50, Hariri has already served three terms since 2009 as premier - a post reserved for a Sunni in Lebanon's power-sharing system. His last cabinet was toppled almost exactly a year ago as protests gripped the country, furious at the ruling elite for decades of state graft and waste.
Hariri, the sole candidate in Thursday's talks, was backed by a majority of parliamentarians.
"I tell the Lebanese who are suffering from hardships to the point of despair that I am determined to work to stop the collapse that is threatening our economy, our society and security," he told reporters.
"IN A LOOP"
Senior U.N. official Jan Kubis warned Lebanon's "traditional political forces" the country would not survive without an effective government to ward off chaos. "Do not count on miracles, foreign elections or external donors – the rescue must start in Lebanon, by Lebanon," he tweeted.
The nomination of Hariri, long aligned with Western and Gulf states, follows weeks of political wrangling that has delayed a deal on a new government. He was backed by his own Future Movement, the Shi'ite Amal party, Druze politician Walid Jumblatt's party and other small blocs.
The heavily armed Shi'ite group Hezbollah, which took part in Hariri's last coalition cabinet, did not nominate anyone but said it would seek "to maintain the positive climate."
Together with its allies, including Amal and Aoun's party, they hold a majority in parliament.
Lebanon's two main Christian blocs did not nominate Hariri. The Free Patriotic Movement, led by Aoun's son-in-law, said a veteran politician could not lead a technocrat government.
Its rival the Lebanese Forces, the second biggest Christian bloc and a staunch Hezbollah opponent, also declined to name him.
Former colonial power France has tried rallying Lebanon's sectarian leaders but has been frustrated by the apparent lack of urgency or progress.
Hariri has presented himself as the "natural candidate" to build a cabinet that can revive the French roadmap, which lists long-neglected reforms and calls for resuming IMF talks.
Many in Beirut remained skeptical that any change would come. Even in the Tariq al-Jadida district where the Hariri family holds sway, Lebanese were weary after a year of turmoil had turned public uproar into despair.
"Even if they bring a divine prime minister down from the sky, a government in Lebanon would not work, because fundamentally, there's no state," shop owner Adnan said.
In Beirut's Hamra street, Marwa Hodeib said she felt stuck in a loop. "I think we might all leave the country and they'll still be here."
(Additional reporting by Laila Bassam, Dominic Evans and Alaa Kanaan; Writing by Dominic Evans and Ellen Francis; Editing by Kevin Liffey, William Maclean) ((Ellen.Francis@thomsonreuters.com;))