Speaking to reporters at the White House, Trump said he expected to do "very well" against the suit, adding that he had an "absolute right" to make the declaration.
"I think in the end we're going to be very successful with the lawsuit," Trump said. "I actually think we might do very well, even in the 9th Circuit, because it's an open and closed case."
A group of 16 states, including California, New York and Colorado, filed a lawsuit Monday against Trump's emergency declaration. The lawsuit, which was filed in federal court in San Francisco, alleges Trump's declaration is unconstitutional.
All the states involved in the lawsuit have Democratic attorneys general.
Using a broad interpretation of his executive powers, Trump declared an emergency last week to obtain wall funding beyond the $1.4 billion Congress approved for border security. The move allows the president to bypass Congress to use money from the Pentagon and other budgets.
Trump's use of the emergency declaration has drawn bipartisan criticism and is expected to face numerous legal challenges. Democrats are planning to introduce a resolution disapproving of the declaration once Congress returns to session and it is likely to pass both chambers. Several Republican senators are already indicating they would vote against Trump — though there do not yet appear to be enough votes to override a veto by the president.
Sen. Lisa Murkowski, a Republican from Alaska, would not explicitly say Tuesday whether she would support a resolution of disapproval if one came before the Senate. But she made clear she was worried about the precedent that could be set by Trump going around Congress to fund the wall.
"I'll be very direct. I don't like this. I don't like this. I think it takes us down a road, and with a precedent, that if it's allowed, that we may come to regret," said Murkowski, who said she supports efforts to bolster security at the border but is concerned about an erosion of checks and balances.
A top White House adviser said Sunday that Trump was prepared to issue his first veto if Congress votes to disapprove his declaration of a national emergency. Stephen Miller told "Fox News Sunday" that "the president is going to protect his national emergency declaration."
Trump argued Tuesday that the wall was needed to "stop drugs and crime and criminals and human trafficking." He has repeatedly sought to paint a dire picture of conditions at the border, though illegal border crossings are down from a high of 1.6 million in 2000.
After weeks spent battling with Congress over border funding and what constituted a wall versus a fence, Trump said, "I can call it a barrier, but I think I don't have to do that so much anymore, we'll call it whatever we want."
Democrats quickly seized on the move as an example of executive overreach. The office of House Speaker Nancy Pelosi, D-Calif., issued a press release Tuesday that stated: "No one is above the law. Republicans must join Democrats to uphold the Constitution and stand with the American people — against the President's brazen assault."
Earlier Tuesday, Trump singled out California for its lead role in the suit, seeking to link the state's high-speed rail project to his plan for the wall.
On Twitter, Trump claimed the "failed Fast Train project" was beset by "world record setting" cost overruns and had become "hundreds of times more expensive than the desperately needed Wall!"
The estimated cost for a San Francisco-to-Los Angeles train has more than doubled to $77 billion. That's about 13 times more than the $5.7 billion Trump sought unsuccessfully from Congress to build the wall.
Last week, California Gov. Gavin Newsom said the rail project "as currently planned, would cost too much and take too long." He said the state would focus on completing a shorter segment in the state's Central Valley while seeking new funding sources for the longer route.
Associated Press writer Becky Bohrer contributed from Juneau, Alaska.
This story corrects that the 16-state lawsuit was filed in federal court, not the 9th U.S. Circuit Court of Appeals.