The US president has vetoed resolutions in Congress to block the sale of $8.1bn (£6.5bn) worth of weapons to Saudi Arabia and the United Arab Emirates.
Donald Trump said the three resolutions “would weaken America’s global competitiveness” and damage its relationship with its allies.
It comes after both chambers of Congress voted to prevent the sale.
Some members of Congress said they feared the weapons could be used on civilians in the Yemen conflict.
They have condemned Saudi Arabia’s role in Yemen as well as last year’s murder of Saudi journalist Jamal Khashoggi.
The Senate would hold a vote within days on whether to override Mr Trump’s veto, Republican Majority Leader Mitch McConnell said on Wednesday.
But analysts say it is almost certain that the Senate will not have the necessary two-thirds majority to do so.
It is the third time Mr Trump has used his veto power since taking office.
In May, the White House issued a national emergency declaration to bypass legislators and push through the weapons sale.
At the time Mr Trump said he made the decision because of the threat posed by Iran.
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But the move sparked fierce opposition on Capitol Hill from those who feared the weapons could be used against civilians in Yemen by Saudi-led forces.
Lawmakers including some Senate Republicans also said there was no legitimate reason to bypass Congress.
However, in issuing his veto Mr Trump suggested that barring the sale of US weapons could prolong the conflict in Yemen and that “without precision-guided munitions, more – not fewer – civilians are likely to become casualties”.
He also insisted that Saudia Arabia and the UAE were a “bulwark against the malign activities of Iran and its proxies in the region”.
Tensions between the US and Iran have risen sharply since Mr Trump took the US out of a deal between Iran and world powers to curb Iran’s nuclear programme.
The US says it downed at least one Iranian drone last week and has blamed Iran for a series of attacks on tankers in the Gulf. Iran shot down a US drone in June.
Trump uses veto to unblock $8bn weapons sale to Saudi Arabia}