Trump made the declaration in response to a question during a joint press conference with Romanian President Klaus Iohannis.
U.S. President Donald Trump publicly confirming for the first time his commitment to NATO's Article 5, which requires members of the alliance to defend each other if they come under attack.
"Certainly we are there to protect and that's one of the reasons that I want people to make sure we have a very, very strong force, by paying the kind of money necessary to have that force", Trump added.
The White House has praised Romania as among the countries meeting the president's call for alliance members to increase their defense spending.
How mutual aid works: Article 5 says an attack on one is an attack on all.
The president's endorsement of NATO's collective defense mechanism came weeks after he put members of the alliance on edge during his first trip overseas.
"I'd put it in the "better late than never" category", said Jeff Rathke, a retired career foreign service officer at the Center for Strategic and International Studies.
During the presidential campaign Trump said the US might not come to the defense of nations that hadn't yet met that goal.
But Trump declined to make the same statement during his speech at North Atlantic Treaty Organisation headquarters in Brussels last month, when he scolded North Atlantic Treaty Organisation leaders for failing to meet the alliance's defense spending guideline of 2% of GDP. "We Europeans have to take our fate into our own hands".