"I am committing the United States to Article 5," Trump said at Friday's press conference, referring to the alliance's principle that an attack on one NATO nation is an attack on them all.
"And certainly we are there to protect," Trump added, saying this is why the US is "paying the kind of money necessary to have that force."
"Yes, absolutely I would be committed to Article 5," he concluded.
But Trump declined to make the same statement during his speech at NATO headquarters in Brussels last month, when he scolded NATO leaders for failing to meet the alliance's defense spending guideline of 2% of GDP.
Appearing with Romanian President Klaus Iohannis on Friday, Trump also reiterated his call for NATO members to meet the guideline — along with his claim that NATO members should repay what he regards as underpayments from previous years.
That didn't seem to bother Iohannis, who noted that Romania was the first country under Trump's administration to "step up to 2 percent of GDP for defense spending."
"I'm very glad that due to your strong leadership NATO decided to go against terrorism," the Romanian president said. "Your involvement made so many nations conscious of the fact that we have to share the burden inside NATO."
Ahead of Trump's comments Friday, Democrats had slammed the President for failing to commit to Article 5 while at NATO, as well as his comments during the campaign that the alliance was "obsolete."
"While it is important that senior officials such as the vice president, secretary of state and secretary of defense reiterate that commitment, explicit endorsement -- and the absence of an endorsement -- has meaning," seven House Armed Services Democrats, including ranking member Adam Smith of Washington, wrote in a letter Friday.
But Sen. Tom Cotton, an Arkansas Republican and ally of Trump in the Senate, argued Trump was just misunderstood.
"It's a good thing that President Trump made explicit what he plainly meant in Brussels last month: the United States stands by the collective security guarantee of NATO Article 5," Cotton said in a statement. "But make no mistake: uttering magic words does not deter aggressors like Vladimir Putin. Only the credible threat of military force does. And until Democrats and our European allies get serious about funding our common defense, deterrence in Europe will remain dangerously weak."