The Suwon District Court determined that Lee, a member of the Unified Progressive Party, led a conspiracy calling for armed insurrection to overthrow the government, the Korea Times reported.
It is the first time that a sitting member of Parliament was convicted of plotting an armed uprising to overthrow a democratically elected government, South Korea's Yonhap News Agency reported.
Lee was found guilty of sympathizing with North Korea, violating South Korea's anti-communist National Security Law, Yonhap reported.
The court suspended Lee's right to vote and other civic rights for 10 years after his release from prison.
The court said Lee led the underground group Revolutionary Organization to stage an uprising, an allegation he has denied.
The court also found Lee and another indicted UPP member defied the anti-communist law by singing communist songs and having banned documents in their possession, the Times said.
The Justice Ministry petitioned the Constitutional Court to dissolve the minority UPP after Lee was arrested in September, saying its activities went against the ideals of the constitution.