The court Sunday overruled a lower court's conviction of a man surnamed Chung who had an affair with a bar hostess during divorce proceedings, a court official told AFP Monday.
The lower court had given Chung, 57, and the hostess a suspended six-month prison sentence, a ruling upheld by an appeal court.
Adultery is punishable by up to two years in jail in South Korea.
The supreme court ruled that Chung and his wife "had no intention of continuing their married life" and therefore sex with a new partner did not constitute an offence.
Chung filed in early 2007 to end his marriage of 25 years and the couple decided to live apart while proceedings were under way. But his wife laid charges against him after learning he had had sex with the hostess.
The constitutional court is meanwhile considering whether to scrap the adultery law altogether following a petition from top actress Ok So-Ri, who says it infringes personal freedoms.
The constitutional court has rejected three previous appeals against the 54-year-old law on the grounds that social morality may be weakened.