The 41-year-old's decision to quit after less than three years in office dealt a new blow to Western governments still in shock from Britain's vote to leave the European Union and the election of outsider Donald Trump as US president.
Underscoring the financial risks that heavily indebted Italy faces, Moody's changed its outlook on the country's bond rating to negative from stable, saying prospects for much-needed economic reform had shrunk after Italians rejected Renzi's proposals to revise the constitution and streamline parliament.
Renzi tendered his resignation to President Sergio Mattarella, who said he would consult with political parties to decide the next steps. He asked Renzi to carry on in a caretaker capacity until a solution is found.
After the consultations, which will begin on December 8 at 1700 GMT and end on December 10 afternoon, Mattarella is widely expected to ask a member of Renzi's cabinet, or a politician from his Democratic Party, to try to form a new government.
Elections are due in 2018 but many politicians are calling for them to be held earlier.