"I said it was obsolete," Trump declared at the White House. "It's no longer obsolete."
The about-face came during a joint appearance with NATO's secretary general in the East Room. Trump's original stance, which he voiced as a candidate, had caused unease among some US allies, who worried a retreat from NATO would embolden Russia's territorial ambitions in Europe.
Trump said Wednesday he'd changed views after NATO's leaders assured him the group would turn its attention to combating groups like ISIS going forward.
"The secretary general and I had a productive discussion about what more NATO can do in the fight against terrorism," Trump said. "I complained about that a long time ago, and they made a change and now they do fight terrorism."
Trump's allegation that NATO just recently began combating terror isn't accurate; the alliance has played a central role in Afghanistan for more than a decade. Its involvement in that war came after the United States invoked NATO's Article 5, the provision calling for collective defense, after the September 11, 2001 terror attacks.
Since then, NATO troops have deployed to Afghanistan by the thousands in a bid to stabilize the country and combat terror groups based there.
Member countries have also pushed for better intelligence sharing to combat terrorism, which NATO has sought to facilitate through a new Joint Intelligence and Security Division.
That addition, however, came in the middle of last year, and NATO said then that the new division wasn't prompted by Trump's campaign rhetoric, but rather a desire among members to better share information after a string of terror attacks in Europe.
In an interview on CNN's "Erin Burnett OutFront," Democratic Sen. Ed Markey said he welcomed the change as Trump squaring "his philosophy with reality, but added that he thought Trump's frequent shifts were "undermining our role on the world stage."
"It doesn't have any coherence as a foreign policy whatsoever, and it continues to look like they're just making it up week to week," Markey said.
The meeting Wednesday was Trump's first face-to-face meeting with Jens Stoltenberg, the NATO leader. The two will meet again in May when Trump travels to the group's headquarters in Brussels for talks.
Trump has long insisted the US carries too much of the burden within the defense collective, and had angrily lambasted other nations for not spending sufficient amounts on defense. Since taking office, however, Trump has softened his language, saying he'll maintain US commitment to NATO while reiterating its member nations must step up their military financing.
"In facing our common challenges, we must also ensure that NATO members meet their financial obligations and pay what they owe. Many have not been doing that," Trump said Wednesday. "If other countries pay their fair share instead of relying on the United States to make up the difference, we will all be much more secure and our partnership will be made that much stronger."
Trump's position on NATO has been the subject of consternation for European leaders, who worried his dismissals of the alliance could encourage Russia, which NATO has worked to check through troop mobilizations on the bloc's eastern edge.
Trump has voiced support for the group since taking office, however, and Wednesday attempted to assure European nations fearful of Russian territorial aggression.
"They are going to have to fear nothing, ultimately," he said. "Right now there is a fear. There are problems, certainly problems, but ultimately I hope that there won't be a fear and there won't be problems and the world can get along. That would be the ideal situation."