The interest due has jumped because of increased borrowing to finance various stimulus measures to end the country's prolonged economic slowdown in the wake of the global financial crisis, the finance ministry said.
The Yonhap News Agency reported the $18.9 billion interest translates to about $377 for every person in the country.
The national debt last year totaled 443.1 trillion won, up sharply from 309 trillion won in 2008. This year it is expected to top 480 trillion won.
Yonhap quoted experts that the rising debt and interest payments could hamper the South Korean government's efforts to maintain fiscal health in the face of tough economic conditions both at home and abroad.