The agreements included an $US110bn arms deal, which the White House described as the single biggest in US history.
Mr Trump and his wife Melania were greeted in the Saudi capital by King Salman on Saturday morning local time.
The eight-day trip comes as Mr Trump faces uproar at home following his sacking of FBI director James Comey.
It will also take in Israel, the Palestinian territories, Brussels, the Vatican, and Sicily.
US Secretary of State Rex Tillerson said the arms deal was aimed at countering the "malign" influence of Iran.