Nauru's President David Adeang met with Xi in Beijing on Monday as part of his first state visit to China since the decision in January.
China will "provide assistance for Nauru to realise its own sustainable development, with no political conditions," Xi told Adeang, according to a readout published by state broadcaster CCTV.
Xi praised the island state's diplomatic switch as being "in line with the general trend of history".
In a blow to self-ruled Taiwan just days after it held elections, Nauru unexpectedly announced it would no longer recognise the democracy "as a separate country" but "rather as an inalienable part of China's territory".
Nauru -- population 12,500 -- is one of the world's smallest countries.
It had previously recognised China in July 2002 after over two decades of diplomatic relations with Taiwan, before switching back to Taipei in 2005.
China claims democratic Taiwan as part of its territory and has said it will not rule out force to bring the island under Beijing's control.
Taiwan and China have engaged in a diplomatic tug-of-war to lure allies in the Pacific region, offering generous aid packages and assistance in agricultural and educational development.
Xi on Monday said China was "willing to expand practical cooperation with Nauru including in trade, investment and infrastructure construction," and offered to specifically help the South Pacific island with tackling climate change.
CCTV said the two heads of state oversaw the signing of bilateral cooperation documents including one on "jointly building the Belt and Road", a vast global infrastructure project that critics have said mires developing nations in debt.
Proponents praise the initiative for bringing resources and economic growth to less developed countries.
Nauru's January decision was seen as a major coup for Beijing.
The island state was one of the few countries to officially recognise Taiwan on a diplomatic basis.
Taipei said at the time of the switch that Beijing's "inducement of... huge financial aid" was behind the decision.
Only 12 states, including the Holy See, now fully recognise Taiwan.
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