"President realDonaldTrump just approved an Emergency Declaration for the State of Hawaii in support of Hurricane Douglas", FEMA Administrator Pete Gaynor tweeted.
The declaration unlocks federal disaster assistance for the state as it braces for the storm's arrival.
Hurricane Douglas was bearing down on the archipelago at the speed of 30 kilometres (18 miles) per hour as of 11 am local time (21:00 GMT), packing maximum sustained winds of 145 kilometres (90 miles) per hour, according to the latest advisory of the National Hurricane Centre.
A hurricane warning has been declared for the Hawaiian island of Oahu, the advisory says.