Xi's presence at this week's anniversary celebrations provides an important boost for the Kremlin leader, who has portrayed his war in Ukraine as a struggle against modern-day Nazis from the start.
Ukraine and its allies reject that characterisation as a grotesque falsehood, accusing Moscow of conducting an imperial-style invasion.
"The victory over fascism, achieved at the cost of enormous sacrifices, is of lasting significance," Putin told Xi on Thursday.
"Together with our Chinese friends, we firmly stand guard over historical truth, protect the memory of the events of the war years, and counteract modern manifestations of neo-Nazism and militarism."
Xi said the two countries, as world powers and permanent members of the UN Security Council, would work together to counter "unilateralism and bullying" - an implied reference to the United States.
He said they would "jointly promote the correct view of the history of World War II, safeguard the authority and status of the United Nations, resolutely defend the rights and interests of China, Russia and the vast majority of developing countries, and work together to promote an equal, orderly, multipolar, and inclusive economic globalisation".
The two leaders spoke after approaching each other along a red carpet from opposite ends of one of the Kremlin's most opulent halls and shaking hands in front of the cameras.
Each greeted the other as "dear friend".
Xi is the most powerful of more than two dozen foreign leaders who are visiting Moscow this week to mark Thursday's 80th anniversary of the end of World War II.
The celebrations are taking place at a key moment in the war with Ukraine, as Moscow and Kyiv come under US pressure to reach a peace deal.
Ukraine's Foreign Ministry on Tuesday urged countries not to send their militaries to take part in the May 9 parade, saying such participation would go against some countries' declared neutrality in the war.
Xi, whose country is locked in a tariff war with the United States, is expected to sign numerous agreements to deepen the "no limits" strategic partnership that the two countries signed in 2022, less than three weeks before Putin sent his army into Ukraine.
