The king placed the Collar of Abdulaziz Al Saud around Trump's neck at a ceremony at the Royal Court in Riyadh.
The host of the event declared that Trump was being honored for "his quest to enhance security and stability in the region and around the world".
Women in Saudi Arabia, including visitors and foreign dignitaries, are expected to be fully covered in public, including their head and hair, according to the country's religious and legal codes, ABC News reported.
Soon after, Trump tweeted for the first time on global soil as president.
In another development, the Washington Post said a current White House official close to Trump was a significant "person of interest" in the investigation into possible ties between Trump's presidential campaign previous year and Russian Federation.
On the drive to the Ritz hotel where Trump is staying, King Salman rode with the president in the heavily armored presidential limousine nicknamed "the Beast".
Trumpets played and soldiers stood at attention as the president walked in.
"I'm very happy to see you", King Salman said, greeting the president.
The two men were then expected to have a bilateral meeting.
Joining Trump inside was a long parade of advisers, including Secretary of State Rex Tillerson, National Security Adviser H.R. McMaster, Commerce Secretary Wilbur Ross, chief of staff Reince Priebus, chief strategist Steve Bannon, senior advisers Ivanka Trump and Jared Kushner, Press Secretary Sean Spicer, and Trump's former bodyguard, Keith Schiller.
White House officials hope the trip, complete with images of the accompanying pomp and pageantry of a president overseas, will help Trump recalibrate after one of the most hard stretches of his young presidency.
Trump's account is delivering a diplomatic message. It says that it's "Great to be in Riyadh, Saudi Arabia" and says Trump is "Looking forward to the afternoon and evening ahead".
It's his first stop on a busy, nine-day trip.
Melania Trump wore a black pantsuit with a golden belt and did not cover her head for the arrival, consistent with custom for foreign dignitaries visiting Saudi Arabia.
U.S. President Donald Trump and first lady Melania Trump wave as they arrive in Riyadh, Saudi Arabia, May 20, 2017.
Trump said in a 2015 tweet that, while many people applauded Michelle Obama's decision, the people of Saudi Arabia "were insulted".
The White House confirmed the draft was authentic, but cautioned the president had not yet signed off on the final product.
Melania is accompanying Trump on a five-country tour that also includes stops in Israel and Italy.
The president's stop in Saudi Arabia kicks off an ambitious worldwide debut.
He'll participate later in an arrival ceremony at the Saudi Royal Court and a luncheon banquet, followed by a bilateral meeting with the king, a signing ceremony and a meeting with the Deputy Crown Prince.
Trump and Salman were expected to oversee the signing on Saturday afternoon of multiple agreements between American and Saudi companies, including between the Saudi oil giant Aramco and various United States firms.
The nation offered Trump an elaborate welcome.
Billboards featuring images of Trump and the king and emblazoned with the motto "Together we prevail", dotted Riyadh's highways, and Trump's hotel was bathed in red, white and blue lights and, at times, an image of the president's face.
Whereas Melania's fitted high-waisted leather skirt and cream sweater with the feminine details, such as ties at the sleeves, won Mrs. Trump raves on Friday from social media, the views about Melania's choice of a black jumpsuit are still flowing into Twitter and other social media platforms as the photos make the rounds online.
Iran and Syria were not invited to the summit, and they are not part of a regional military alliance that Saudi Arabia is establishing to fight terrorism.
White House officials have said they consider Trump's visit, and his keynote address, a counterweight to President Obama's debut speech to the Muslim world in 2009 in Cairo.
Trump remains popular in the Gulf, where leaders hope he'll take a harder line on Iran than Obama did.
Trump flew to Riyadh overnight on Air Force One.
The centerpiece of Trump's visit to Saudi Arabia is a speech Sunday at the Arab-Islamic-American summit.