The veto would be the first of Trump's presidency. Twelve Senate Republicans sided with Democrats on the issue.
Mr Trump said that, since 1976, a total of 56 national emergencies have been declared but Congress has not vetoed any of them.
Shortly after the veto was official, House Speaker Nancy Pelosi announced that the vote to potentially override the veto would occur on March 26, saying "House Republicans will have to choose between their partisan hypocrisy and their sacred oath to support and defend the Constitution".
Also in the room for the veto signing Friday were parents of children who were killed by immigrants who entered the country illegally, as well as Vice President Mike Pence, DHS Secretary Kirstjen Nielsen and Attorney General William Barr.
In declaring an emergency on February 15, Trump cited drug smuggling across the border from Mexico (misleading), the drop in crime in El Paso, Texas, after a partial border barricade was built there (not true) and reports of women kidnapped, bound with tape and trafficked into the United States across unguarded sections of the border (no evidence exists of this). Trump was surrounded in the Oval Office by supporters who offered profuse thanks and frequent applause.
"The world continues to be a unsafe place and we'll continue in the war against terrorism", said the senator. The 12 GOP senators, including the party's 2012 presidential nominee, Mitt Romney of Utah, joined the dissent over the emergency declaration order that would enable the president to seize for the wall billions of dollars Congress meant to be spent elsewhere.
"I think actually a national emergency was designed for a specific objective like this, so we have a great case", Trump said.
Despite the embarrassing defections, Trump's grip on the party remains strong and the White House made it clear that Republicans resisting Trump could face political consequences. "To me, border security is national security".
"I put no pressure on anybody", Trump said. He noted, correctly, a spike in the number of people coming to the border to claim asylum. His signature ended a 35-day partial shutdown of the federal government, which had been triggered by a showdown after Congress refused to appropriate the $5.7bn he requested to start construction at the border.
Congress declined and the result was the longest shutdown in USA history. Trump had campaigned for president promising Mexico would pay for the wall.