A law requiring South Korea's citizens to use their real names on websites has been struck down by a panel of judges.
The country's Constitutional Court said the rule restricted freedom of speech and undermined democracy.
The requirement was introduced in 2007 as part of efforts to tackle cyber-bullying.
But the judges said users had switched to overseas sites where they continued to conceal their identity, putting local services at a disadvantage.
There had also been complaints that the system had made it easier for cybercriminals to commit identity theft.
The "internet real-name system" stipulated that news media sites with more than 100,000 visitors a day had to record the real identities of visitors who had posted comments. The users' resident registration numbers were used as verification.
The move was introduced following a spate of posted comments describing fabricated sex scandals, made-up plastic surgery operations and other untrue controversies involving celebrities.
Local reports had linked a number of suicide attempts to the problem.
The idea behind the law was that users' details could be disclosed if the victims of malicious reports wanted to sue for libel or infringement of privacy.
But the eight judges unanimously voted against the law saying the public gains achieved had not been substantial enough to justify restrictions on individuals' rights to free speech.
They said that the policy discouraged people from criticising influential people and groups because of fears they would be punished.
"Expressions under anonymity or pseudonym allow (people) to voice criticism on majority opinion without giving into external pressure," the court said.
"Even if there is a side effect to online anonymity, it should be strongly protected for its constitutional value."
The ruling comes months after China's Twitter-like microblogging services were ordered to ensure their members had registered accounts under their real names.
The authorities had complained about "unfounded rumours" being posted on the service.
The services face being shut down if users fail to comply.
But the country's leading provider, Sina, has admitted it has had problems verifying the millions of users' details involved.