Prosecutors also charged Kim Ki-Jong, a nationalist activist, with assaulting a foreign envoy and obstruction of duty.
Police say Mr Kim wanted to highlight his opposition to joint military exercises with the US.
Mr Kim denies attempting to kill ambassador Mark Lippert, who received 80 stitches after the attack in March.
The envoy was targeted at a breakfast event in Seoul and spent several days in hospital, recovering from gashes to his face and hand.
Doctors said Mr Lippert's injuries could have been life-threatening if the cuts had been any deeper.
Mr Kim was tackled at the scene and arrested. Under South Korean law, the trial has to start within 14 days.
He could face life in prison if convicted of attempted murder, or even the death penalty, though that is rarely used in South Korea.
Prosecutors are also examining whether Mr Kim can be charged under a controversial National Security Law, which bans any praise or assistance for North Korea.
However, Mr Kim insists he was acting alone, rather than on the orders of Pyongyang.
Many anti-US activists such as Mr Kim favour reunification with North Korea and regard the massive US troop presence in South Korea as an obstacle to their aims.
The attack on Mr Lippert prompted rallies and prayer sessions in his support and in support of US-South Korea relations.
Kim Ki-jong's militant past
A South Korean man who attacked the US ambassador to Seoul with a knife, slashing his face and arms, has been formally charged with attempted murder.