SYDNEY—Australia’s highest court found that newspapers and television stations that post articles on Facebook Inc.’s platform are liable for other Facebook users’ comments on those posts, a ruling that could prompt traditional publishers to rethink how they engage with social media.

The High Court of Australia determined that media companies, by creating a public Facebook page and posting content on that page, facilitated and encouraged comments from other users on those posts. That means the media companies should be considered publishers of the comments and are therefore responsible for any defamatory content that appears in them, according to a summary of the judgment from the court.

Media companies, including News Corp Australia and Nine Entertainment Co., owner of the Sydney Morning Herald, called for legislators to change Australia’s laws to protect them from liability, saying the ruling essentially makes media companies responsible for other people’s comments even when they are unaware the comments have been posted.

News Corp and Nine Entertainment had appealed to the High Court, Australia’s equivalent to the U.S. Supreme Court, after a lower court in the Australian state of New South Wales ruled last year that the media companies are responsible for third-party comments. Wednesday’s ruling upholds the lower court ruling.

“We are obviously disappointed with the outcome,” said a Nine Entertainment spokesperson. “It will have ramifications for what we can post on social media in the future.”

Michael Miller, executive chairman of News Corp Australia, said: “This highlights the need for urgent legislative reform and I call on Australia’s attorneys general to address this anomaly and bring Australian law into line with comparable Western democracies.”

News Corp Australia is a subsidiary of News Corp, which also owns Dow Jones & Co., publisher of The Wall Street Journal.

A Facebook spokesperson declined to comment on the court’s decision, but pointed out the company changed its comment moderation policy earlier this year. It introduced a feature that effectively allows media companies to turn off comments on their Facebook pages, which could protect media outlets from defamation allegations but also make it more difficult to promote their articles and earn advertising revenue.

Some legal experts expressed concerns about the ruling. Jason Bosland, an associate professor at the University of Melbourne’s law school, said it could hinder the ability of media companies to promote important public-interest journalism. Mr. Bosland said even moderating comments in real time wouldn’t be enough to completely shield newspapers from liability, because the ruling effectively means media companies are responsible for people’s comments the instant they are posted.

“I think it chills the capacity for the media to produce stories which provoke public discussion, and that’s exactly what you want the media to do,” he said. “You want them to be able to put up stuff online and you want them to be able to encourage people to engage in a civilized public debate.”

The case was initially brought by Dylan Voller, who was detained in a juvenile detention center and became the subject of media attention. Articles about Mr. Voller that media outlets posted on Facebook drew comments from other Facebook users falsely accusing him of serious crimes.

Wednesday’s ruling only addressed the issue of whether traditional media companies could be considered publishers of third-party comments on social media. The case will now continue in the lower courts, where the media companies could argue other legal defenses.

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Mr. Voller’s lawyers said media companies have a strong commercial imperative to increase engagement on their posts, and that they assisted the publication of the third-party comments by posting articles in the first place.

“This decision put responsibility where it should be: on media companies with huge resources, to monitor public comments in circumstances where they know there is a strong likelihood of an individual being defamed,” his lawyers, O’Brien Criminal & Civil Solicitors, said in a statement.

Australia is already considered to be a defamation hot spot because of plaintiff-friendly laws that make it easier to win defamation lawsuits compared with other jurisdictions. In Australia, free speech isn’t protected by the constitution and a defendant such as a newspaper must prove that a statement is true. In the U.S., where the First Amendment protects freedom of speech, it is the plaintiff’s responsibility to prove an alleged defamatory statement is false.

A reform of Australia’s laws, aimed at giving media companies stronger defenses in court, is already under way and some Australian states have approved changes. A second stage of reforms, including looking at whether online platforms should be held liable for comments from third-party users, is also being considered.

U.S. law broadly exempts social-media companies like Facebook from legal liability for what people post on the site, though some U.S. politicians have called for changes.

The relationship between Australian news publishers and social-media giants gained global attention earlier this year when Australian lawmakers passed a law effectively requiring tech companies to pay news publishers for content.

At one point, Facebook removed news from its platform for Australian users—though it restored it a few days later following a public uproar and after the Australian government agreed to make small changes to the law. Facebook afterward signed deals with publishers, including News Corp, for content. Alphabet Inc.’s Google also signed deals with publishers, including News Corp.

Write to Mike Cherney at mike.cherney@wsj.com