Napolitano "will stand down within the next few hours with a long list of achievements to his credit," Renzi said as he marked the end of Italy's six-month rotating presidency of the European Union.
The prime minister's announcement follows reports earlier today that Napolitano would sign his resignation letter on Wednesday and leave the Quirinale Palace shortly afterwards.
The 89-year-old president on Monday met with Renzi to say his farewells and discuss key issues affecting Italy, including terrorism, as well as the upcoming elections in Greece.
Pietro Grasso, an anti-mafia prosecutor and head of the Italian Senate, will be appointed acting president until a successor is appointed, Il Sole 24 Ore reported.
Lawmakers and regional deputies have 15 days after the president’s resignation to begin choosing his replacement, with January 28th earmarked as the likely date for voting to commence.
Napolitano began an unprecedented second term in April 2013 after bickering politicians failed to agree on a new head of state. At the time, he made clear his intention to step down as soon as Italy was on a more stable political path.
Rumours of Napolitano’s imminent departure began circulating late last year, with names of numerous political heavyweights cited as the possible new president.
Calls for Italy to elect its first female head of state took a blow on Monday, when former foreign minister Emma Bonino announced she was undergoing treatment for lung cancer. The ex-European commissioner and long-serving MP had won wide support as a possible candidate.
Despite a push for an outsider to take over at the Quirinale, members of the “old guard” of Italian politicians continue to dominate press speculation. They include Romano Prodi, twice prime minister, and Pierluigi Bersani, former centre-left leader who stepped down after a dismal election performance in 2013.