People found the northern fur seal resting on the sand near Sunset Beach on Oahu's North Shore. It was emaciated, underweight and weak.
It's the first time on record that a wild fur seal has come to Hawaii, said David Schofield, a marine mammal response coordinator at the National Oceanic and Atmospheric Administration.
Hawaii's only native seal is the endangered Hawaiian monk seal. Elephant seals have made their way over from California in the past, but only very rarely, Schofield said.
"We were all pretty surprised," he said.
It's not clear how the female visitor, a young adult, got so far south.
Some theorize it may have hitched a ride on a cargo ship or even rode over on some debris from Japan's 2011 tsunami, but Schofield said both were unlikely. The seal may have wandered off and gotten caught in a strange current that brought her to the islands.
"The bottom line is we'll never know," he said.
NOAA officials took her to the Honolulu Zoo to be cared for and to prevent her from spreading any diseases to Hawaiian monk seals.
Northern fur seals are known to carry the measles-like virus morbillivirus, which could devastate the already shrinking Hawaiian monk seal population. Hawaiian monk seals haven't been exposed to diseases like this and don't have any resistance because they've been living in isolation from other seals for so long.
NOAA officials hope to send the northern fur seal to the Marine Mammal Center in Sausalito, Calif., which has extensive experience nursing sick and injured seals back to health.
"We'll do our very best to provide it with the best care that we can," said Jeffrey Boehm, the center's executive director. The center hopes to return the seal to the wild, he said.