“The president emphasized that Italy will remain one of the United States' closest and strongest allies and an indispensable partner,” the White House said in a statement.
Obama made the call from Air Force One on a flight from Washington to Tampa, Florida, thanking Renzi “for the close friendship and partnership the leaders enjoyed” during his tenure as prime minister.
The outgoing Democratic president, who will be succeeded by Republican Donald Trump next month, had praised the center-left Renzi's proposed reforms to streamline parliament and the electoral system.
Obama hosted Renzi at his final state dinner at the White House on October 18th.
After Renzi, 41, suffered a crushing defeat in a referendum Sunday on constitutional reform, he presented his resignation to Italian President Sergio Mattarella on Monday.
But Mattarella asked Renzi to postpone his resignation until parliament approves the 2017 budget, in the interest of easing political uncertainty in the country and across Europe.