Ms Ok said the law violated her right to choose her relationships
South Korean prosecutors have demanded an 18-month jail term for a popular actress who admitted breaking the country's strict laws on adultery.
Ok So-ri had sought to overturn the 50-year old legislation, which carries a maximum jail sentence of two years.
She said it was an infringement of human rights and amounted to revenge.
But in October the constitutional court ruled for the fourth time that adultery must remain a crime, saying it was damaging to social order.
Ms Ok has admitted having an affair with a well-known pop singer and her husband, Park Chul, is said to be seeking "a severe sentence".
She blamed her infidelity on a loveless marriage to Mr Park, also an actor, and launched a legal challenge against the adultery law itself.
But the court ruled that the adultery law did not violate the right to "sexual self-determination and privacy" and that the available punishment was appropriate.
"Society still recognises that adultery damages social order," said the court.
"The punishment of a two-year jail term is not excessive when comparing it to responsibility."
Ms Ok's lawyers have said the legislation "has degenerated into a means of revenge by the spouse, rather than a means of saving a marriage".
The Korean Times says that in the past three years about 1,200 people have been indicted annually for adultery, but very few have been jailed.
The case has created a sensation in South Korea, say correspondents, where many have denounced what they see as an archaic law.