Lippert was attacked while he was on his way to a breakfast meeting in the Sejong Cultural Institute in central Seoul on Thursday, CNN reported.
The report showed the ambassador after the attack with his left hand covered in blood.
According to police reports, the attacker was armed with a razor, which he used to slash the ambassador's wrist and face.
Following the attack, Lippert was rushed to hospital, where his injuries were reported not life threatening.
Kim Young-man, a spokesperson for the Korea Council for Reconciliation and Cooperation, which was hosting the breakfast meeting, apologised for the lack of security at the function.
According to reports, the assailant, who was wearing traditional Korean clothes, was tackled by police and security forces.
State Department spokesperson Marie Harf confirmed the ambassador had been attacked while giving a speech in Seoul Thursday morning, and said that the South Korean embassy is coordinating with local law enforcement on the attack.
"We strongly condemn this act of violence. The ambassador is being treated at a local hospital. His injuries are not life threatening," she said.
Lippert is a longtime friend and confidant of the President Barack Obama, who has been a member of his inner circle since the President's time as senator.
According to National Security Council spokesperson Bernadette Meehan, Obama called Lippert to wish him well.
"The President called the US Ambassador to the Korea, Mark Lippert, to tell him that he and his wife Robyn are in his thoughts and prayers, and to wish him the very best for a speedy recovery," Meehan said in a statement.