The president, in an effort to pile pressure on one of America’s closest allies over the rising cost of oil, called on the kingdom to increase its defence budget or face an uncertain future.
Speaking at a campaign rally in Mississippi, he said: "I love the king, King Salman, but I said: 'King, we're protecting you. You might not be there for two weeks without us. You have to pay for your military, you have to pay."'
Crude oil prices are at a four-year high with the November mid-term elections looming, and Mr Trump claimed just last week that oil producers were “ripping off the rest of the world”.
The president did not on elaborate when he spoke to the Saudi monarch, but reports suggested the two leaders last shared a telephone call over the weekend.
State-run Saudi Press Agency said the pair discussed "efforts to maintain supplies to ensure the stability of the oil market and ensure the growth of the global economy".
There was no immediate reaction in Saudi Arabia to Mr Trump's remarks.
"Opec nations are, as usual, ripping off the rest of the world and I don't like it," he said. "Nobody should like it. We defend many of these nations for nothing, and then they take advantage of us by giving us high oil prices. Not good. We want them to stop raising prices. We want them to start lowering prices and they must contribute substantially to military protection from now on."
The US and Saudi Arabia maintain close military ties, including in counter-terror operations against Isis in the Middle East.
Following a 2017 summit in Riyadh, both countries agreed to increase cooperation on maritime security, military preparedness, arms transfers, and cyber security.
The US state department said American security ties had allowed the kingdom to foil “numerous terrorist attempts against Saudi and foreign targets” and “deter external attacks”.
Saudi Arabia is also America’s largest foreign customer of military hardware.
Riyadh has worked to cultivate warm relations with the US president after having rocky moments with his predecessor, Barack Obama.
Saudi Arabia welcomed Mr Trump for his first overseas trip as president.
The US administration – and in particular his son-in-law Jared Kushner – has sought a close relationship with King Salman's son, Mohammed bin Salman, the country's crown price and next in line to the throne.
In July, Mr Trump tweeted without evidence that Saudi Arabia would increase its production "maybe up to 2,000,000 barrels" a day.
Saudi Arabia currently produces some 10 million barrels of crude oil a day. Its record is 10.72 million barrels a day.
Additional reporting by agencies
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