Xi told Trump in the phone call on Monday that Taiwan's return to mainland China is "an integral part of the post-war international order," according to the Chinese foreign ministry.
A White House official confirmed that the call happened on Monday morning but offered no details of the call.
The conversation came after Japanese Prime Minister Sanae Takaichi recently said Japan's military could get involved if China were to take action against Taiwan, the self-governing island that officials in Beijing say must come under its rule.
Japan is an important ally of the US in the region.
China has since rebuked Takaichi's remarks, and China-Japan relations have plunged.
Xi in the phone call said China and the US, which fought together during the war against fascism and militarism, should "jointly safeguard the victory of World War II".
The US has taken no side on the sovereignty of the self-governed island but is opposed to the use of force to seize Taiwan.
It is obligated by a domestic law to provide sufficient hardware to the island to deter any armed attack.
Trump has maintained strategic ambiguity about whether he would send US troops in case of a war in the Taiwan Strait.
His administration has urged Taiwan to increase its military budget.
