The decision came on a day that attorneys from four states were in courts challenging Trump's executive order.
A federal judge on Friday put a nationwide block on US President Donald Trump's week-old executive order temporarily barring refugees and nationals from seven countries from entering the United States.
The Seattle judge's temporary restraining order represents a major setback for Trump's action, although his administration could still appeal the ruling this weekend and have the policy put back into effect.
Judge James Robart, a George W. Bush appointee, made his ruling effective immediately on Friday, suggesting that travel restrictions could be lifted straight away.
Shortly after the ruling, US Customs and Border Protection told airlines to board travellers affected by the ban. The US State Department is working with the Department of Homeland Security to work out how Friday's ruling affects its operations, a spokesman told Reuters, and will announce any changes affecting travellers as soon as information is available.
The Justice Department made no immediate decision on an appeal but said in a statement it would determine its next steps after reviewing the written order.
Robart's ruling followed an earlier decision by a federal judge in Boston declining to extend a temporary restraining order allowing some immigrants into the United States from countries affected by Trump's three-month ban.
The state of Hawaii on Friday also filed a lawsuit alleging that the order is unconstitutional and asking the court to block the order across the country, while US District Judge Leonie Brinkema in Alexandria, Virginia ordered the federal government to give the state a list of "all persons who have been denied entry to or removed from the United States" by Thursday.
The challenge in Seattle court was brought by the state of Washington and later joined by the state of Minnesota. The judge ruled that the states have legal standing to sue, which could help Democratic attorneys general take on Trump in court on issues beyond immigration.
Washington's case was based on claims that the state had suffered harm from the travel ban, for example students and faculty at state-funded universities being stranded overseas.
Judge Robart probed a Justice Department lawyer on the "litany of harms" suffered by Washington state's universities, and also questioned the administration's use of the Sept. 11, 2001 attacks on the United States as a justification for the ban.
Robart said no attacks had been carried out on US soil by individuals from the seven countries affected by the travel ban since that assault. For Trump's order to be constitutional, Robart said, it had to be "based in fact, as opposed to fiction."