Seven out of nine judges ruled that outlawing abortion was unconstitutional -- votes from six judges were needed to overturn the ban.
Lawmakers now have until December 31, 2020 to revise the law. Termination of pregnancy after 20-weeks will remain illegal.
Previously, women who had abortions in South Korea could face up to a year in prison and can be fined up to two million won ($1,780), while doctors or healthcare workers who helped terminate a pregnancy could be jailed for up to two years.
While prosecutions were rare, they were not unheard of.