Hundreds of students waving Cambodian and Chinese flags greeted Xi alongside officials at the airport before his motorcade sped into town for an audience with the royal family and later Prime Minister Hun Sen.
Cambodia has long been a strident supporter for Beijing's communist rulers, who have lavished the poor country with aid and loans.
In recent years it has become a thorn in the side for its neighbours hoping to present a unified front against Beijing's island building in the South China Sea.
But Cambodia's unwavering support for China has scuppered regional efforts to jointly rebuke Beijing.
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Ahead of his trip, a leading Cambodian newspaper published an article signed by Xi that praised the Southeast Asian nation for coming to its defence over the sea row.
"When China acted to safeguard its territorial sovereignty and maritime interests and was devoted to resolving related disputes through peaceful negotiation, Cambodia did not hesitate to speak out to uphold justice," the Chinese president wrote in the article published by Cambodia's Rasmei Kampuchea newspaper.
Large portraits of the Chinese leader and Cambodia's King Norodom Sihamoni were erected along the streets of the capital Phnom Penh ahead of the two-day visit, which is Xi's first there as president.
At least 28 pacts, including deals to boost the sale of Cambodian agricultural products to China, will be signed during a meeting between Xi and the premier later Thursday.
China is Cambodia's top foreign investor and has given the country billions of dollars in grants and low-interest loans during Hun Sen's 31-year rule.
When Xi visited Cambodia in 2009 as vice president, the two countries signed 14 bilateral agreements worth $1.2 billion dollars in aid and loans to Cambodia.
Hun Sen regularly praises Beijing's "no-strings-attached" aid, compared to help from the United States and European Union which is often accompanied by calls to address corruption and human rights abuses in his country.
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