Trump made the remark to reporters in London, where he is attending a meeting of North Atlantic Treaty Organization leaders, according to Reuters.
The comment comes as the US has demanded South Korea contribute more to the upkeep of 28,500 American troops in the country, with negotiations over a new cost-sharing deal currently under way.
"It can be debated. I can go either way. I can make arguments both ways," Trump said when asked if it was in the US national interest to station troops in the Asian ally. "But I do think this, I think if we're going to do it, they should burden-share more fairly."
The Trump administration has reportedly demanded a fivefold increase in Seoul's contribution to nearly $5 billion next year.
Reports have said Washington is preparing to reduce its forces in South Korea should the country reject its request.
Trump also said Tuesday that South Korea agreed to pay nearly $500 million last year for US "protection," and that the US now wants additional commitments, according to Reuters.
South Korea agreed to pay $870 million this year, an increase of 8.2 percent from last year. (Yonhap)