She disagreed with the government's assertion that the blocked Twitter users' right to free speech does not apply in this case because Trump was acting as a private individual through his @realDonaldTrump account.
White House press secretary Sarah Huckabee Sanders and former communications director Hope Hicks are dismissed as defendants in the suit, but social media director Daniel Scavino remains a defendant because he routinely accesses Trump's Twitter account.
The US Department of Justice, which represented the President in the case, argued Mr Trump's own First Amendment rights allowed him to block people with whom he did not wish to interact.
Buchwald issued a "declaratory judgment" on behalf of the plaintiff, writing, "no government official - including the President - is above the law, and all government officials are presumed to follow the law as has been declared".
The president uses his account, which has 37,600 tweets and 52.2 million followers, to "take measures that can only be taken by the president as president", the ruling reads.
"This is a great precedent we have set for politicians that decide to use social media as a town hall to be held accountable to the rules of the First Amendment", Gu said.
Comedy Central's 'The Daily Show previews a pop-up library exhibiting President Trump's tweets at Union Station on October 19, 2017 in Chicago, Illinois.
A request for comment sent to a White House media contact email account was returned as undeliverable.
The wording of the judge's ruling is unambiguous on this point, stating "the viewpoint-based exclusion of the individual plaintiffs from that designated public forum is prescribed by the First Amendment".
"No government official - including the President - is above the law", wrote Buchwald for the U.S. District Court for the Southern District of NY.
The letter says that while Trump might have been offended by the post, and others critical of the president, the tweets are a protected form of free speech, and the users shouldn't be punished by the government for their content.
Neither Trump nor the White House immediately responded to Wednesday's ruling, on Twitter or elsewhere.
A Justice Department spokeswoman said "We respectfully disagree with the court's decision and are considering our next steps".
She was blocked past year after tweeting to Mr. Trump: "To be fair you didn't win the WH: Russia won it for you".
In what seems to us like the weakest part of the decision, Buchwald concludes that the "interactive space" around Trump's tweets qualifies under this standard.
Knight Institute v. Trump - was brought to Buchwald's bench by the Knight First Amendment Institute at Columbia University last July.
The judge did not issue an order against Trump, and the plaintiffs did not ask for one.
Jameel Jaffer, the Knight Institute's executive director, said in a release that his organization was pleased. "As long as they remain blocked, 'the Individual Plaintiffs can not view the President's tweets; directly reply to these tweets; or use the @realDonaldTrump webpage to view the comment threads associated with the President's tweets while they are logged in to their verified accounts, '" she wrote, quoting from a court document. A Twitter-wide ban cuts users off from the feeds of public figures with even more finality than a block.