US envoy to Seoul Mark Lippert was taken to hospital after the attack that followed a lecture Thursday morning, reported the Yonhap news agency.
Lippert is the United States ambassador to the Republic of Korea. He previously held senior positions in the Department of Defense from May 2012 until September 2014.
Televised pictures showed Lippert bleeding heavily. A suspect was arrested, though officials have not made a statement on his identity or motives.
According to South Korean media, witnesses at the scene said the attacker shouted his name, Kim, several times during the attack. "Kim" reportedly has a criminal record and was last in prison in 2010 for throwing a concrete sculpture at the Japanese ambassador.
Reports say "Kim" used a razor blade to attack the ambassador and shouted "No drills for war," an apparent reference to ongoing South Korea-US military exercises.