Hariri won the backing of a majority of parliamentarians in consultations with Aoun on Thursday.
The prime minister-designate, who has previously led three governments in Lebanon, stepped down almost a year ago under pressure from unprecedented protests against economic hardships.
Hariri vowed Thursday to quickly form a new Cabinet of nonpartisan specialists.
He will lead a government that must pull Lebanon out of its deepest crisis since the 1975-1990 Civil War.