Gentiloni, a member of Prime Minister Matteo Renzi's centre-left Democratic Party (PD), will replace Federica Mogherini, who was set to take on her new role as foreign policy chief in Brussels on Saturday.
"Congratulations to Paolo Gentiloni, new foreign minister," Deputy Foreign Minister Lapo Pistelli said on Twitter.
An official confirmation of his appointment was expected later on Friday, with Italian daily La Repubblica saying the swearing-in ceremony in front of President Giorgio Napolitano would take place at 1700 GMT.
Gentiloni, who will be 60 years old next month, is known as one of Renzi's close allies. A former journalist who studied political science, he held the post of communications minister under Prime Minister Romano Prodi between 2006 and 2008.4
He has been a member of the lower house of parliament's foreign and European affairs commission since February 2013.