The nine-member bench ruled by seven to two says the 1953 law was unconstitutional in a judgement handed down on Thursday.
The court held that adultery was a private matter in which the State should not intervene.
According to the presiding judge, Park Han-Chul “even if adultery should be condemned as immoral, state power should not intervene in individuals’ private lives.”
It was the fifth time the Apex court had considered the constitutional legality of the legislation which had made South Korea one of the few non-Muslim countries to regard marital infidelity as a criminal act.
In the past six years, close to 5,500 people have been formerly arraigned on adultery charges – including nearly 900 in 2014.
The numbers had been falling, with cases that end in prison terms increasingly rare.
In 2004, 216 people were jailed under the law in 2004. That figure had dropped to 42 by 2008, and since then only 22 have found themselves behind bars, according to figures from the state prosecution office.