The Foreign Secretary tweeted that "China must respect the Falklands' sovereignty" after Argentinian president, Alberto Fernandez, met with China's President Xi Jinping on the fringes of the Beijing winter Olympics.
According to a statement on London's Chinese Embassy website, the two leaders spoke of their "deep friendship" and Argentina signed up to China's Belt and Road infrastructure initiative, a state-backed campaign for global influence.
But they also signed an agreement in which China reasserted its support for Argentina's claim to the Falklands, while Fernandez backed Xi's one-China policy, which claims Taiwan as its own.
The statement said Argentina should be able to "fully exercise its sovereignty over the Malvinas (Falklands) Islands issue."
But Truss said: "We completely reject any questions over sovereignty of the Falklands.
"The Falklands are part of the British family and we will defend their right to self-determination. China must respect the Falklands' sovereignty."
Chen Weihua, a journalist for China Daily, an English-language newspaper owned by the Chinese Communist Party, replied to Truss to say: "But it's okay for UK to challenge China's sovereignty in the South China Sea by sending navy vessels?
"At least China has not sent its navy near the Malvinas, or what you call the Falklands."
Xi has also met with Russia's president, Vladimir Putin, in recent days, where the leaders pushed back against US pressure and declared their opposition to any expansion of Nato - a key issue in the current tensions on the Ukrainian border.
In a joint statement, they criticised "interference in the internal affairs" of other states and in a thinly veiled reference to the West, said: "Some forces representing a minority on the world stage continue to advocate unilateral approaches to resolving international problems and resort to military policy."
China has increasingly shown support for Moscow in its dispute with Ukraine that threatens to break out into armed conflict.
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