0448 GMT March 22, 2019
Trump declared in the Oval Office that he was "proud" to sign the veto.
It came after he suffered an embarrassing defeat on Thursday when senators, including fellow Republicans, voted to terminate his declaration of an emergency on the Mexican border.
Surrounded by law enforcement officials, senior aides and people who have lost loved ones to cross-border crime, Trump said the veto reaffirming his power to get the funds without Congress was to "defend the safety of all Americans."
"The mass incursion of illegal aliens... has to end," he said. "People hate the word 'invasion' but that’s what it is... Our immigration system is stretched beyond the breaking point."
Trump's emergency declaration allows him to secure funding for construction of border walls after he failed to get authorization from Congress.
Opponents, who accuse Trump of executive overreach and overhyping the problem on the border, could now use court challenges to halt the emergency measure.
Nancy Pelosi, the Democratic speaker of the House of Representatives and a leader in the fight to prevent Trump's wall plans, called Trump's action a "lawless power grab."
“The House and Senate resoundingly rejected the president’s lawless power grab, yet the president has chosen to continue to defy the Constitution, the Congress and the will of the American people," she said in a statement.
The Senate would have to vote to override the veto as well, requiring more Republicans than the original 12 to sign on, which is unlikely to happen.
US Attorney General William Barr said the president's emergency declaration was legal.
It is being challenged in court as an unconstitutional usurpation of Congress' power of the purse.
Trump has made border security an over-arching domestic issue in his presidency and says it will remain at the center of the agenda in his 2020 reelection bid.
Although there has been a surge in arrival of families and children at the border, overall apprehensions at the boundary are down substantially from a decade or more ago.
Researchers have said that immigrants are less likely to commit crimes, despite Trump repeatedly linking immigration and crime.
The president has said he wants a wall to prevent immigrants from crossing into the United States illegally. Democrats deny there is an emergency at the border, saying border crossings are at a four-decade low.
Most Republicans support Trump's position that the border is out of control. However, there were defections in Thursday's Senate vote by Republican senators angered at what they see as Trump's improper seizing of power over the government purse strings – a role reserved for the legislature.
Trump thanked Republican senators who voted for his declaration in a Twitter post earlier on Friday. "Watch, when you get back to your State, they will LOVE you more than ever before!" he said.
AFP and Reuters contributed to this story.