China's foreign ministry describes Xi's trip as the "largest-scale diplomatic activity between China and the Arab world" raising concerns in Washington DC.
Saudi Arabia and China have showcased deepening ties with a series of strategic deals during a visit by President Xi Jinping.
King Salman signed on Thursday a "comprehensive strategic partnership agreement" with Xi, who received a lavish welcome in a country forging new global partnerships beyond the West.
News reports estimated that the deals between the two countries are worth as much as $30 billion.
The two leaders also agreed to hold a meeting between the nations by turns every two years.
In an op-ed published in Saudi media, Xi said he was on a "pioneering trip" to "open a new era of China's relations with the Arab world, the Arab countries of the Gulf, and Saudi Arabia".
China and Arab countries would "continue to hold high the banner of non-interference in internal affairs, (and) firmly support each other in safeguarding sovereignty and territorial integrity", he wrote.
Before his meeting with King Salman on Thursday, the Chinese leader was greeted with a handshake by 37-year-old Crown Prince Mohammed bin Salman, de facto ruler of the world's biggest oil exporter, at Yamamah Palace.
Arab leaders also began to converge on the Saudi capital ahead of a summit with Xi, the leader of the world's number-two economy, who will hold separate talks with the six-member Gulf Cooperation Council before leaving on Friday.
'Raising pace' of cooperation
After Xi's arrival on Wednesday, Saudi state media announced 34 investment agreements in sectors including green hydrogen, information technology, transport and construction.
The official Saudi Press Agency did not provide details but said two-way trade totalled $80 billion in 2021 and $27 billion in the third quarter of 2022.
State broadcaster Al Ekhbariya said another 20 agreements worth $29.3 billion were due to be signed on Thursday.
While in Saudi Arabia, Xi also held a bilateral meeting with the Palestinian leader, Mahmoud Abbas, according to photos published by the Palestinian Authority's press office.
Reports said that he would also hold separate talks with other Arab leaders.
Egyptian President Abdel Fattah el Sisi, Tunisian President Kais Saied and Sudan's leader Abdel Fattah al Burhan were all flying in on Thursday.
Iraqi Prime Minister Mohammed Shia Al Sudani, Moroccan Prime Minister Aziz Akhannouch and Lebanese caretaker Prime Minister Najib Mikati have also confirmed their attendance.
China's foreign ministry this week described Xi's trip as the "largest-scale diplomatic activity between China and the Arab world" since the People's Republic of China was founded.
It has not escaped the attention of the White House, which warned of "the influence that China is trying to grow around the world", calling its objectives "not conducive to preserving the international rules-based order".
Washington has long been a close partner of Riyadh, but the relationship is currently roiled by disagreements on a number of issues including energy policy and security.