The Constitutional Court overturned a law that prohibited doctors from informing parents of the sex of the foetus, which it said was "out of step with social changes" and in breach of the rights of medical workers.
South Korea introduced the law in 1987 in order to help prevent abortions to select the sex of the child, which had led to an alarming demographic imbalance in favour of sons over daughters.
Two petitions were filed with the court challenging the law, including one by a doctor who was suspended for six months after informing parents of the sex of their foetus.
"I am concerned that the court's decision could bring sex-selection abortions back into fashion," said Koo Young-Moo, a professor of medical ethics at Ulsan University.
The country has curbed the trend of aborting females, in part with an awareness campaign called "Love Your Daughter." But Koo said this was also partly due to other wider social changes.
"Many married couples choose to have only one baby, regardless of the baby's sex, or none at all," he said.