Trump, whose world views have been rebuked by a section of the Republican establishment, made the comments as he unveiled a partial list of foreign policy advisors who are relatively little known.
"We are paying disproportionately (for NATO). It's too much and frankly it's a different world than it was when we originally conceived of the idea," Trump said in an interview on CNN. "We have to reconsider. Keep NATO, but maybe we have to pay a lot less toward NATO itself."
The comments reinforced Trump's relatively isolationist stance on foreign policy. He has alarmed mainstream Republican foreign policy thinkers with comments denigrating Muslims and Mexican immigrants, and with his vows to tear up international trade deals.
Announcing his foreign policy advisors in an interview with the Washington Post editorial board, he said the United States should not be "nation-building anymore," adding that Washington could not afford to keep funding NATO at current levels.
"We certainly can't afford to do this anymore," he said.
Trump has said he is willing to work more closely with authoritarian Russian President Vladimir Putin, whose military intervention in Ukraine has increased tensions with NATO.
Washington is the biggest contributor to NATO, and U.S. officials have long been critical of its European allies for not spending more.
NATO is planning its biggest build-up in eastern Europe since the Cold War to deter Russia, helped by an increase in planned U.S. spending.
Trump's advisory team includes terrorism expert Walid Phares, energy industry executive Carter Page, international energy lawyer George Papadopoulos, former Pentagon inspector general Joe Schmitz, and former Army Lieutenant General Keith Kellogg.
'Obscure' team
He said he would soon name more people who are helping him shape his foreign policies as part of the team, led by U.S. Senator Jeff Sessions, a Republican from Alabama who endorsed Trump last month.
"Taken as a group, it's a fairly obscure set of individuals," said Michael O'Hanlon, a national security and defence policy specialist at the Brookings Institution, adding that he does not recognise most of the names on Trump's list.
Trump, one of three remaining Republican presidential candidates, has been under increasing pressure in recent weeks to say who advises him on foreign and security matters.
One hundred and twenty Republican national security experts who have served in past presidential administrations have signed a letter saying they cannot support Trump and will work to ensure he is not elected.
Phares told Reuters he began advising Trump on Friday. He previously had served as a national security adviser to 2012 Republican presidential candidate Mitt Romney, who has roundly criticized Trump. Phares said in an email he has not met Trump in person since last year.
The others on the list could not be immediately reached to confirm their role with Trump's campaign.
Schmitz was Pentagon inspector general under President George W. Bush and worked for Blackwater Worldwide, a now-defunct private U.S. security firm whose personnel were involved in a deadly shooting that killed Iraqi civilians in 2007.
Kellogg has considerable military experience, having served as the chief operating officer of the Coalition Provisional Authority, the U.S.-run provisional government imposed on Iraq after the U.S.-led 2003 invasion under Bush. He has worked at CACI International, a Virginia-based intelligence and information technology consulting firm.
Trump has vigorously criticised the invasion. Asked at a news conference about why he chose Kellogg despite his role in Iraq, Trump said: "I don't have to agree with them but I have to hear different opinions."
According to the Post, Papadopoulos previously advised Trump's former rival Ben Carson, who has now backed Trump. The London Center of International Law Practice's Center website lists him as the head of its Center for International Energy and Natural Resources Law & Security.
Page serves as a managing partner of Global Energy Capital, a private energy services company, the Post said.
Sessions is not seen as an influential foreign policy or national security player in the U.S. Congress. He does not serve on the influential Senate Foreign Affairs Committee, although he is a member of the Armed Services Committee.
Last week, Trump said in a television interview on MSNBC, which had been pressing him to name his foreign advisory team, that he relies on his own instinct.
"I'm speaking with myself, number one, because I have a very good brain," he told MSNBC. "I know what I'm doing. ... My primary consultant is myself."
