"The point is: the Greek referendum won't be a derby EU Commission versus Tsipras, but euro versus dracma (sic). This is the choice," Renzi wrote in a tweet referring to Greek Prime Minister Alexis Tsipras.
The Italian leader's comments echoed warnings earlier on Monday from French President Francois Hollande and European Commission President Jean-Claude Juncker that a vote against the bailout package will mean Greece having to leave the euro and revert to its former currency, the drachma.
Tsipras has called the referendum in the hope of securing a convincing no vote he hopes will strengthen his chances of securing a better deal on the conditions - further cuts and raised taxes - attached to the bailout.
Polls suggest a majority of Greek voters oppose further austerity measures but also want to stay in the euro, making the outcome of the vote hard to predict, as well as the consequences of a decision either way.