Judge Naomi Reice Buchwald of the US District Court for the Southern District of NY sided with the argument made by the Knight First Amendment Institute at Columbia University that the president's Twitter feed constitutes a "designated public forum", and that Trump therefore can't block users based on their political views.
The case was brought by The Knight First Amendment Institute at Columbia University on behalf of seven Twitter users who had been blocked by Trump for criticising him or mocking him online.
District judge Naomi Buchwald ruled [PDF] that Trump's blocking of critics from his Twitter account @realDonaldTrump is illegal since it is "properly characterized as a designated public forum" given his public position.
The lawsuit contended that because Trump uses Twitter for a variety of policy announcements, the account is "a designated public forum" that can not exclude people due to their political views.
"I think it's a wonderful decision but I'm sure it's going to be appealed", said Lakier, who noted that courts are beginning to recognize the significance of social media she compared it to the right of access to a public park.
Katie Fallow, a staff attorney at Knight who represented the plaintiffs, said the ruling "should guide all of the public officials who are communicating with their constituents through social media".
The distinction between muting and blocking showed the judge applied a subtle, but sophisticated, understanding of Twitter and social media, said Aaron Caplan, a professor at Loyola Law School of Los Angeles specializing in constitutional law.
The case is Knight First Amendment Institute v. Trump, 17-cv-5205, U.S. District Court, Southern District of NY.
Trump (and his social media director, Dan Scavino) are prohibiting people from the form of speech that is a Twitter reply.
The photo, which some said showed the Pope glaring at Mr Trump, was captioned: "This is pretty much how the whole world sees you". And since Trump is the president, he acts as a government official, meaning that shutting down users' free speech is a violation of the First Amendment. "Sometimes I will see dozens of people RTing an unavailable tweet and saying, 'This is the most disgusting thing I've ever read.' And I'm like, 'Ah jeez I wonder what that's all about.' The guy literally provokes global leaders with his Twitter account and I'm out of the loop on it".
The problem is that Trump also uses his Twitter account to conduct official public business.
The Justice Department and Twitter are yet to comment on the ruling.
The bigger impact here, however, is that this ruling applies to all public officials in the US.
No order to unblock users: Buchwald stopped short of ordering the president to unblock users, saying there was no need to get into a "legal thicket" over the issue.
Justice Department lawyers had argued it was Trump's prerogative to block followers, no different from the president deciding in a room filled with people not to listen to some.
Now the Knight Institute is representing a man in a similar case in Loudoun County, Virginia, where he was denied access to a Facebook page by a local county official.