The ruling came after the Seoul High Court in April increased his prison sentence to seven years from five after finding Yoon guilty of additional charges.
There was no misunderstanding of any legal interpretations in the previous court's ruling, the Supreme Court said on Thursday.
The Supreme Court upheld the appeals court's finding that Yoon was also guilty of fabricating documents and failing to follow the legal process required to impose martial law, which has to be discussed at a formal cabinet meeting, as well as spreading false information to foreign media outlets.
After the ruling, Yoon's lawyers said they would seek a ruling from the Constitutional Court following Thursday's decision.
"We will challenge the constitutionality of this ruling through constitutional review procedures, including a constitutional complaint," a lawyer for Yoon said.
Prosecutors, who had sought a 10-year prison term in the case, accused Yoon of abusing his power and hurting the public.
Yoon, 65, was also sentenced to life in prison in February on charges of masterminding an insurrection tied to his martial law declaration.
Facing seven other trials, Yoon has been in jail since July 2025.
