The verdict for Lee Sang-min on Thursday came a week before a different judge at the same Seoul court rules on whether Yoon's actions amounted to rebellion, a crime for which prosecutors are seeking the death penalty.
Lee, who led the ministry of interior and safety, was convicted for helping carry out the declaration and passing along Yoon's orders to the country's police and fire chiefs to cut water and electricity to news organisations that were critical of his policies.
The instructions were not carried out since martial law was lifted quickly after MPs broke through a military and police blockade at the National Assembly and unanimously voted to lift it.
Lee denied receiving or issuing instructions to cut off utilities to the news organisations, disputing testimony from other officials, including former National Fire Agency commissioner-general Heo Seok-gon, who said Lee conveyed such orders by phone.
Lee also denied conspiring about or planning the martial law declaration.
Judge Ryu Kyung-jin said testimony from other officials, security camera footage and other evidence clearly showed Lee attempted to carry out Yoon's orders and that it was indisputable Lee played an important role in enforcing martial law.
The judge, however, acquitted Lee of lesser charges of abusing his power, citing a lack of evidence.
Lee's legal team did not immediately say whether he would appeal.
Prosecutors had sought a 15-year sentence.
Lee is the second member of Yoon's cabinet convicted in connection with martial law.
Former prime minister Han Duck-soo received a sentence of 23 years in prison but has appealed.
The conservative Yoon has defended martial law as a necessary act of governance against the liberals, whom he described as "anti-state" forces obstructing his agenda with their legislative majority.
He was later removed from office and has been under arrest since July while facing multiple criminal trials, with the rebellion charge carrying the most severe punishment.
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