It follows a case brought on behalf of seven individuals who had been blocked by the president.
Officials using social media accounts were not allowed to silence people who disagreed with them, the judge said. President Trump’s lawyers argued that his Twitter account was a personal one and not subject to those rules. A US Department of Justice official said it was disappointed by the ruling and was exploring next steps.
In its ruling, the appeals court said: “The First Amendment does not permit a public official who utilises a social media account for all manner of official purposes to exclude persons from an otherwise‐open online dialogue because they expressed views with which the official disagrees.” The ruling follows one made in May last year by the Court for the Southern District of New York that found the president’s Twitter account, @realDonaldTrump, constituted a public forum.