US District Judge Timothy Kelly issued a written decision that elaborated on his September 24 emergency order that reinstated access for the journalists.
The news organisations and press freedom groups have accused Trump of unconstitutional retaliation against journalists for coverage he does not like.
Kelly's order last month gave the CNN, MS NOW and Politico journalists access to the White House grounds on a temporary basis.
The judge issued a preliminary injunction on Tuesday that will remain in effect until he makes a final judgment in the lawsuit brought by the news organisations, unless an appeals court lifts his order.
Kelly wrote Trump's public remarks made clear that he banned the news outlets because he considered their reporting biased and inaccurate, and not because of national security concerns as government lawyers later asserted in court.
"Simply put, that is unlawful viewpoint discrimination," the judge wrote, adding that "no one should read this decision as suggesting that President Trump's complaints are meritless."
Past presidents have battled with the press, but Trump's campaign against the media has been unprecedented. He has upended decades of tradition by denying basic access to many major news organisations, launching personal attacks on journalists for their reporting and elevating media outlets seen as more sympathetic to the administration. He also has filed lawsuits against multiple news outlets.
Trump announced the ban in September on his social media platform, saying those news organisations "shouldn't be able to constantly write or report FICTION and LIES when they're covering the President of the United States."
The outlets have said the revocation of their White House passes was unlawful retaliation for their news coverage and accused the Trump administration of manufacturing meritless national security concerns to justify the ban.
The outlets said in a lawsuit the ban violated the US Constitution's First Amendment protections for freedom of speech and a free press, as well as their rights to due process, meaning fair treatment from the government.
The White House on September 24 reinstated the CNN, MS NOW and Politico journalists after the judge decided the ban "likely violated their constitutional due process rights," and ordered their access immediately restored.
The judge, who Trump appointed in 2017 during his first presidential term, also raised doubts about the US Justice Department's national security arguments for justifying the ban, noting that Trump had said the action was due to what the president alleged was false or negative coverage.
Justice Department lawyers have said the outlets endangered national security by publishing classified information, citing stories about US missile stockpiles, construction of Trump's White House ballroom and turnout at the recent Republican midterm convention.
The judge held a hearing on October 8 to hear arguments by the news outlets and the Justice Department over whether to extend the protection for the journalists against being barred from the White House. The Justice Department defended the bans as fair and justified.
"Here we have the unusual situation where the president himself has said he wants to chill speech. He wants to coerce better coverage," Theodore Boutrous, a lawyer for the outlets, told Kelly during the hearing.
"CNN, MS NOW and Politico engaged in apparently indefensible conduct," Justice Department lawyer Michael Velchik said.
