"The order does not offer a sufficient justification to suspend the entry of more than 180 million people on the basis of nationality," the court wrote, referring the combined populations of the six countries.
Under immigration law, the administration was required to make findings that entry of the people in question would be detrimental to the United States but failed to do so, the court said.
Stephen Vladeck, a professor at University of Texas School of Law, said the 9th Circuit provided an easier path for the Supreme Court to keep the travel ban on hold, because it avoided entirely the controversy over Trump's campaign statements.
"It provides a very attractive way to leave the injunction in place without setting broader doctrinal rules about which they may have pause," Vladeck said.
Monday was the deadline for the ban's challengers to respond to the administration's request that the order be allowed to go into effect. The American Civil Liberties Union, which represents people challenging the ban in the separate Maryland suit handled by the 4th Circuit, filed court papers urging the court not to take up the case, saying the order will become moot on Wednesday, 90 days from when Trump issued it.