President Trump said Sunday he has no plans to withdraw U.S. troops from South Korea, potentially assuaging worries in Seoul as Mr. Trump’s end-of-the-month summit with North Korean leader Kim Jong Un approaches.

“I have no plans, I’ve never even discussed removing them,” Mr. Trump said in an interview on CBS News’ “Face the Nation” program. The U.S. president repeated his complaint that maintaining what he said are 40,000 U.S. forces in South Korea—the actual number is about 28,500—is “very expensive.”

Mr....