SEOUL—The U.S. and South Korea signed a one-year deal outlining the shared costs of their military alliance on Sunday, removing a potential distraction ahead of the second summit between President Trump and North Korean leader Kim Jong Un scheduled for late this month.

Seoul will pay roughly $920 million this year for the 28,500 U.S. military personnel stationed in the country, according to officials from both countries. That represents an increase of about 8% from what Seoul paid in 2018. South Korea foots about half of the...