A popular actress has raised a petition questioning the law which treats adultery as a criminal offence.
Ok So-Ri filed the petition in February, claiming the 54-year-old adultery law infringes on the right of individual choice in sexual relations,
The constitutional court has rejected three previous appeals on the grounds that social morality may be weakened.
"Based on today's hearing, a nine-member panel will make a decision later. We don't know when it will come," spokesman Kim Bok-Gi told AFP.
The hearing was attended by legal experts, justice ministry officials and lawyers, he said. It was not known whether Ok or her husband were present.
Ok, 39, is awaiting trial for adultery after her actor husband Park Chul sued her and two men for allegedly having affairs with her. They are a hotel chef of Italian descent and a Korean pop singer.
She admitted having an affair with a singer but put the blame on her loveless marriage to Park.
Park also filed a divorce suit against her. According to South Korean law, when a spouse accuses a partner of adultery he or she must also file for divorce.
The current law sets a maximum two-year prison sentence for an adulterer or adulteress. Ok's petition rekindled debate over whether adultery should be a criminal offence.
Some women's groups contend the law must be maintained to protect female rights in a traditionally male-dominated society.
A magazine survey quoted by newspapers showed that nearly 68 percent of South Korean men and 12 percent of women confessed to having sex outside marriage.
Critics say the law breaches an individual's right to sexual autonomy and has become outdated.
Court data shows the number of people jailed for adultery has declined sharply over the years. Last year only 47 were jailed while 592 were given suspended terms.
Some lawmakers are pushing for a bill to abolish the criminal punishment of adultery.
Two liberal judges including Do Jing-Gi filed separate petitions last year, saying adultery should be handled under civil and divorce laws.
Do said it should be seen as a violation of trust and of sexual morality between wife and husband.
"Sex is a natural human desire and it is inappropriate to restrict it by law," he said.