The Russian president was greeted by around 200 well-wishers waving Russian flags as he arrived, almost an hour late, for a tour of his country's pavilion at the world fair.
A smiling Putin was welcomed by Italian Prime Minister Matteo Renzi, with whom he will have bilateral talks followed by a joint press conference.
He then travels on to Rome where an audience with Pope Francis is expected to focus on Ukraine amid fears a fragile ceasefire agreed in February is falling apart.
Renzi was one of the G7 leaders who signed up to Monday's warning that the major powers "stand ready to take further restrictive measures to increase cost on Russia should its actions so require".
The statement reflected concern about a recent flare-up in fighting in eastern Ukraine, where the West accuses Russia of providing game-changing military support to pro-Moscow rebels who control parts of two Russian-speaking regions.
Ukraine this week said Russian aid allowed separatist forces in the east to establish a 42,500-strong fighting force.
Putin says any Russians fighting alongside the rebels are volunteers "answering a call of the heart".
€30 billion trade ties
Italy has long had an important economic relationship with Russia and political ties were sufficiently close prior to the eruption of the Ukraine crisis for Russia's Baltic neighbours to object to an Italian, Federica Mogherini, being named the EU's foreign policy chief last year amid claims she would be too soft on Moscow.
According to the foreign ministry, Italy is Russia's third-biggest trading partner after China and Germany. Exchanges between the two countries were worth just over €30 billion last year, down by over a billion euros from 2013 due to sanctions and their knock-on effect on the Russian economy.
Renzi has said he will not be lecturing the Russian leader during their talks on Wednesday morning, while stressing that Italy stands fully behind international demands that Moscow ensure the respect of a second ceasefire between Kiev and the rebels that was agreed in Minsk in February.
"I have nothing to explain to him that he does not know already: the compass, the guiding star is the Minsk agreement," Renzi said at the end of the G7, striking a notably different tone to Barack Obama.
The US president said Putin was wrongheadedly trying to recreate the glories of the Soviet empire.
Foreign Minister Paolo Gentiloni said on Monday that Italy wanted to see more dialogue with Russia on issues other than Ukraine and increased business ties.
"The fact that we are going through a period of tension related to Ukraine... should not prejudice the need for dialogue with Russia on many international issues and the existence of economic relations which, in sectors not subject to sanctions, should not have any brake applied to them," he said.
"Russia is trying to diversify its economy because of the (low) oil price and this can be an additional opportunity for Italian companies in infrastructure and other areas."
Still a player?
Analysts say Putin's meetings with Renzi, recently-elected president Sergio Mattarella and the pope will play well to his domestic audience, giving the impression he remains a player on the global stage.
The Vatican has been criticized, notably by the Rome-aligned Greek Catholic minority in Ukraine, for failing to overtly criticize Moscow's actions in Ukraine.
But the crisis has also caused a historic rapprochement between the Vatican and the Russian Orthodox church to stall since Putin last visited the Pope in November 2013, dashing hopes the Argentinian pontiff could become the first Pope to visit Moscow since the 11th Century schism between Eastern and Western Christianity.
After his visit to the Vatican, Putin is expected to call on long-standing friend Silvio Berlusconi, the media tycoon and disgraced former Italian Prime Minister. The two men, who have regularly holidayed together, last met in Milan in October, carousing together until 3:00am while Berlusconi was still serving a community service order for tax fraud.