Lebanon's newly elected President, Michel Aoun, has asked Saad Hariri to form a new government.
Mr Hariri is a leading Sunni Muslim political figure and previously served as prime minister between 2009 and 2011.
His return to the premiership was expected as part of a deal that saw Mr Aoun, a Christian former general, elected president earlier this week.
That ended a political stalemate in Lebanon that had lasted 29 months.
The country faces security challenges related to the civil war in neighbouring Syria and is struggling to cope with the influx of more than one million refugees.
There are also major problems with the economy, infrastructure and basic services.
Speaking at the presidential palace after his nomination, Mr Hariri told reporters he hoped to form a government of national accord quickly, and promised a "new era for Lebanon" after two years of deadlock.
"We, the government, will get to work to resolve the economic, environmental, security and political issues the Lebanese face," he said.
"Our duty toward the Lebanese is to work quickly to protect our nation from the burning fires around it, to protect it in the face of terrorism, and to provide mechanisms to face the burdens of the Syrian displaced people. And to restore hope and trust in our youth of a better future."
Mr Hariri had objected to Mr Aoun's candidacy because he is an ally of the powerful Shia Muslim Islamist Hezbollah movement, members of which are accused by an international court of being behind the assassination of his father Rafik in 2005.
The speaker of parliament, Nabih Berri of the Shia Amal movement, had reportedly initially opposed the appointment of Mr Hariri as prime minister.
But on Thursday, Mr Berri said he would endorse him and co-operate with his efforts to form a government.
Hezbollah MPs meanwhile chose not to endorse any candidate for the premiership.