"My experience of government finishes here," said a downcast Renzi after accepting a defeat of almost 60-40 percent over his constitutional reform bid.
The result, coming on the coattails of Brexit and Donald Trump's US election win, has plunged Italy into a period of political uncertainty and cast a shadow over the short- and long-term future of the eurozone's third-largest economy.
After a first meeting with President Sergio Mattarella to discuss his departure, Renzi posted a link on his Facebook page defending his record since he came to power in February 2014.
Renzi, 41, was expected to formally tender his resignation to Mattarella after a final cabinet meeting set for 6.30 pm (1730 GMT).
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Mattarella will then be charged with brokering the appointment of a new government or, if he is unable to do that, ordering early elections.
Initial market reaction was subdued.
The euro briefly sank to a 20-month low as investors fretted that political instability could scupper efforts to resolve a long-running banking crisis, and over the possibility of an election that could see anti-EU parties challenge for power.
Traders were reassured in part by the result of Europe's other crucial vote this weekend, which saw Austria reject a far-right candidate for president.
"The next steps are far from clear for Italy and traders are not panicking yet", said Craig Erlam, senior market analyst at Oanda.
Some analysts said the referendum could come to be seen as a landmark moment.
Holger Schmieding, at the Berenberg private bank, said the risk that Italy could choose to leave the euro, while still remote, had increased.