The President confirmed he had signed a weapons deal with the Saudis worth $109.7 billion, predicted to grow to a $380 billion Saudi investment within 10 years, during his first trip abroad since his Inauguration.
Mr Trump’s Press Secretary Sean Spicer said the deal was positive news for American employment and the economy.
Yet a February Gallup poll found that Saudi Arabia is one of the least liked countries by Americans, only slightly less than Russia.
The deal would also appear hypocritical after the President publicly accused the Saudis of masterminding the attacks on the Twin Towers in 2001.
After his election, Mr Trump said Saudi Arabia should be banned from exporting oil to the US, and has accused the country of killing gay people and enslaving women.
Amnesty International accused the President of a “glaring omission” of human rights on the leaders’ agenda, and called for the US to stop selling arms to the Saudis to prevent the nation’s violation of international law via air strikes in Yemen and killing civilians.
“This brazen disregard for human rights and humanitarian law will only serve to further embolden states in the Gulf and around the globe in their pursuit of ‘security’ at the expense of people’s basic rights,” said Margaret Huang, executive director of Amnesty International USA.
According to Kristine Beckerle, the Yemen researcher at Human Rights Watch, the sale puts Americans at risk and exposes US officials to legal liability for “aiding and abetting coalition war crimes”.
But in recent months the Saudi Deputy Crown Prince Mohammed bin Salman has developed a relationship with Mr Trump, seven months before the President would arrive in Riyadh with his wife, Melania Trump, to kick off his trip overseas and give a speech on how to combat radical Islam.
His national security adviser H R McMaster promised the speech would be “direct but inspiring”.
King Salman gave Mr Trump the Collar of Abdelaziz Saud, the highest civilian honour and named after the founder of modern Saudi Arabia, who struck a lucrative deal to give their oil fields to US companies. Mr Trump bowed as he received it, the same move he attacked former President Barack Obama for making in 2012.
Saudi Arabia was not included on the original list of seven Muslim-majority countries which were impacted by Mr Trump’s executive order to temporarily ban immigration and refugees to the US.
Mr Trump will also meet Israeli leaders on Monday to discuss an Israeli-Palestine peace deal. Critics have voiced concerns that as the US has signed a deal worth almost half a trillion dollars with the Saudis to supply them with arms, which could be used against Israel, the US is unlikely to cross Saudi Arabia to defend its ally.