The court, in a five-to-four vote, ruled that article 92-6 of the military criminal act was constitutional. The act does not explicitly refer to same-sex intercourse but bans "anal intercourse" or "any other indecent act" during service.
It was the fourth time since 2002 that the court upheld the law, which carries a prison term of up to two years.
"The world has advanced toward the elimination of LGBT discrimination, but the minds of Constitutional Judges have not taken a single step forward," said Centre for Military Human Rights Korea head Lim Tae-hoon in a statement.
South Korea court upholds ban on gay sex in the military - BBC News