Lawmaker Lee Seok-ki was sentenced to 9 years behind bars on Monday on charges of incitement and violating National Security Law.
The Seoul High Court also imposed a 7-year ban on serving in parliament.
Back in February, a lower court sentenced Lee to 12 years in jail, but the judge on Monday commuted it, citing a lack of of evidence that Lee conspired to stage a rebellion or had the means to overthrow the South Korean government.
Lee, a member of the left-wing United Progressive Party was arrested last September and found guilty of plotting an armed rebellion against the South Korean government in February-- making him the nation's first sitting lawmaker to be convicted of treason since the 1980s.
Prosecutors had demanded a 20-year prison term for Lee, claiming he led an underground group of 130 members with alleged links to North Korea.
Lee has always denied the charges, with his party saying he is a victim of a "witch hunt."
However, it was confirmed that the lawmaker did praise North Korea and did speak about how South Korea should prepare for an attack from Pyongyang.
Lee claims he had organized the groups solely for peaceful purposes.
Connie Lee, Arirang News.