The USD 110 billion deal for Saudi purchases of US defence equipment and services came at the start of an eight- day foreign tour that will also take Trump to Jerusalem, the Vatican and meetings with leaders in Europe.
The US president was given a warm welcome in the oil-rich kingdom - a mood in sharp contrast to Washington where pressure is building after fresh claims over his team's alleged links to Moscow.
Air Force One had barely taken off when it was announced late yesterday that James Comey, the former FBI chief fired by Trump, had agreed to testify publicly about Russian interference in the US elections.
Reports also emerged that Trump had called Comey "a nut job" and that the FBI had identified a senior White House official as a "significant person of interest" in its probe of Russian meddling.
The president and first lady Melania Trump were welcomed by Saudi King Salman as they disembarked at King Khalid International Airport in Riyadh this morning.
Trump and his wife, who dressed conservatively in black but did not cover her hair as Saudi women are required to do, walked side-by-side to the tarmac where they both shook hands with the 81-year-old king.
After a welcoming coffee ceremony, Trump and his entourage were brought to the royal court where the president was awarded the Collar of Abdulaziz Al Saud, Saudi Arabia's highest civilian honour.
The arms sale agreement was just one of a series of deals to be announced during the visit, with US conglomerate General Electric saying it had also signed agreements and memorandums of understanding worth $15 billion.
Trump held talks with Salman and was to meet the kingdom's two powerful crown princes today, before giving a speech on Islam to leaders of Muslim countries tomorrow.
For Riyadh the visit is an opportunity to rebuild ties with a key ally, strained under Trump's predecessor Barack Obama who Sunni Arab Gulf states suspected of a tilt towards their Shiite regional rival Iran.
A more muted focus on human rights should also please Washington's traditional Sunni Gulf allies, analysts say.
Shortly after arriving Trump took to Twitter to express his delight at being in the kingdom.
"Great to be in Riyadh, Saudi Arabia. Looking forward to the afternoon and evening ahead," Trump wrote.
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