Diab's appointment came almost two months after his predecessor Saad Hariri handed in his resignation amid nationwide protests that have gripped Lebanon since October, reports Efe news.
"I have been informed by President Aoun of the results of the parliamentary consultations and I have been assigned to form the next government," Diab said during a press conference on Thursday at the Baabda presidential palace here.
"I thank the President and the lawmakers for the confidence they have given me."
The Sunni-Muslim academic won 69 votes, more than half of the 128-seat Parliament.
Diab, who is teaching at the American University of Beirut, held the Education portfolio between 2011-2014 under the cabinet of Najib Mikati.
Lebanon's Shia Hezbollah and Amal groups nominated Diab, as well as their allied Christian-majority party the Free Patriotic Movement.
The new premier has not received any support from the Sunni bloc that has either abstained or voted for jurist and diplomat Nawaf Salam, who got 13 votes.
According to the Lebanon's Constitution, the president has to consult with the parliamentary blocs, who nominate different candidates for the job.
The premier must always be a Sunni Muslim, as per the Constitution, while the presidency is reserved for a Christian Maronite and the speakership of the lower house is occupied by a Shia Muslim, according to the power-sharing deal between the three main religious sects.
Lebanon has been gripped by street protests since October 17, prompting the Hariri to resign after 12 days on October 29.
The demonstrators are calling for the departure of the country's ruling elite whom they accuse of mismanagement and corruption.
( With inputs from IANS )