The man, identified only by his last name Lee, was arrested July 16. The arrest was announced Friday.
"We appreciate the quick action of the Korean authorities in identifying and arresting a suspect," U.S. Embassy Spokeswoman Nida Emmons told CNN.
According to investigators, Lee, who is in his early 30s, is unemployed and lives a secluded life with little social interaction.
He left his post on the White House site on July 8, police said. When the U.S. Embassy noticied South Korean officials, they tracked Lee down using the I.P. address of the computer used to leave the post.
On his computer, police said they found a draft of the post.
Lee denies the allegation.
Recovering from attack
The threat didn't affect Lippert's activities, Emmons said. He is already under increased protection after he was stabbed March 5
during an event organized by the Korean Council for Reconciliation and Cooperation, which advocates peaceful reunification between North and South Korea.
Shortly before Lippert was supposed to give a speech, the attacker slashed him in the face and jaw.
The ambassador suffered a gash from his right cheekbone to his lower jaw that required 80 stitches. That wound measured 10 centimeters (4 inches) long and 2.5 centimeters (1 inch) deep, but there was no serious facial nerve damage, said Dr. Jung Nam-shik of the Yonsei Severance Hospital.
Lippert also suffered five cuts in his left arm and hand, but was not expected to have permanent damage to his arm function.
The alleged attacker, Kim Ki-Jong, was charged with attempted murder.