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The ruling, delivered by Seoul Central District Court Judge Jee Kui-youn, was lighter than the death penalty prosecutors sought at the trial’s final hearing in January.
Jee said in the verdict that Yoon led an insurrection and committed acts to subvert the country’s constitutional order.
The court added that Yoon “took the lead in planning the crime and involved a large number of people,” and that it was “difficult to see any sign of remorse from the defendant, who also refused to appear in court.”
Yoon had previously reportedly refused to appear in court for questioning last year.
The court also found Yoon had ordered South Korea’s military to capture individuals during the martial law declaration, including the current president Lee Jae Myung.
Yoon also had the intention to “paralyze” the country’s parliament by deploying troops to blockade the National Assembly and arrest key politicians, Jee said.
Five others were also sentenced, including former defense minister Kim Yong-hyun, who was sentenced to 30 years in prison.
The defendants may appeal the ruling within a week.
The sentence follows a separate ruling on Jan. 16, when Yoon was given a five-year prison term for attempting to obstruct his arrest after he was impeached and suspended from office.
Other senior officials have been sentenced over the failed martial law bid. Former Prime Minister Han Duck-soo was sentenced to 23 years in prison, while former Interior Minister Lee Sang-min received a seven-year sentence.
Yoon was the first sitting South Korean president to be arrested in an operation involving over 3,000 police personnel that involved a standoff with presidential security agents.
Martial law attempt
Yoon reportedly maintained his innocence, arguing the declaration was within his constitutional authority and was intended to “safeguard freedom and sovereignty.”
He imposed South Korea’s first instance of martial law in 44 years during a late-night address on Dec. 3, claiming that the then-opposition Democratic Party of Korea was engaging in “anti-state activities” and colluding with “North Korean communists.”
Troops were deployed to the country’s National Assembly, while soldiers and police clashed with protesters outside the compound.
Television footage showed special forces breaking windows to gain access to the chamber, while parliamentary staffers used furniture to barricade doors.
Then-defense minister Kim Yong-hyun also reportedly ordered troops to “pull people inside the National Assembly building outside.”
But the martial law order was overturned within three hours, after 190 of the 300 National Assembly lawmakers gathered in the chamber and unanimously voted to do so. Yoon eventually lifted martial law about six hours after announcing it.
He was impeached 11 days later and removed from office on April 4, 2025.