The Korean site of the Canada-based company - slogan: "Life is short. Have an affair" - went online in the middle of last month, garnering close to 50,000 subscribers in its first week.
Under a 1953 statute that criminalises adultery, an unfaithful spouse in South Korea can receive a prison sentence of up to two years for having an affair.
The Korea Communications Standards Commission (KCSC), a largely government-appointed body, said the website was an incitement to immorality.
"In light of the legal spirit of relevant laws aimed to protect healthy sexual morals, marriage bonds and family life, KCSC decided to block access to the site which incites adultery," the commission said in a statement.
"There is a great danger of this site spreading bad behaviour and seriously undermining legal order by aiding or abetting in adultery," it added.
South Korea is the second Asian country to block Ashley Madison after Singapore, which barred access in November for similar reasons.
The website is no stranger to Asia, having already launched in Japan, India and Hong Kong.
Ashley Madison CEO Noel Biderman believes the South Korean law is "hopelessly outdated" but still heeded legal advice not to attend the South Korea launch in person.
He insists that his website simply facilitates an activity that is universal and crosses all social and geographical boundaries.
"Infidelity is present in Asian culture, in the same way that it is present in every other culture in the world," he told AFP in a recent telephone interview from New York.
South Korea's adultery law is not much of a deterrent, and conviction usually results in a suspended sentence rather than actual jail time.
As an offence, it can only be prosecuted on complaint, and any case is closed as soon as the plaintiff drops the charge.
Whereas 216 people were given prison terms under the law in 2004, that figure had dropped to 42 by 2008.
But it remains on the statute books, despite half a dozen referrals for review to the country's Constitutional Court, and there is no great groundswell of opinion to have it removed.
In 2011, a Christian pastor was jailed for 18 months for having a decade-long affair with a woman whose wedding he had officiated at, after her husband named them both in an adultery complaint.