U.S. District Judge Clark Waddoups also ruled that American Samoans should be issued new passports reflecting his ruling. The disclaimer on their passports currently reads: "The bearer is a United States national and not a United States citizen."
"This court is not imposing 'citizenship by judicial fiat,'" Waddoups said in his decision. "The action is required by the mandate of the Fourteenth Amendment as construed and applied by Supreme Court precedent."
American citizens are defined as people "born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside."
American Samoa became a U.S. territory in 1900, but those born there are only recognized as U.S. nationals, preventing them from being able to vote, run for public office or sponsor family members for immigration to the U.S.
Its status separates itself from other U.S. territories such as Puerto Rico, Guam, the Northern Marianas and the U.S. Virgin Islands.
In 2016, the Supreme Court declined to hear a case from the U.S. Court of Appeals for the District of Columbia Circuit that ruled the Constitution doesn't confer citizenship to those born in American Samoa.
The lawsuit was brought last year by three people -- John Fitisemanu, Pale Tuli and Rosavita Tuli -- who were born on the cluster of islands southeast of Hawaii and currently reside in Utah. They claimed they faced restrictions from traveling abroad and were subject to fees that don't apply to American citizens.
It was not clear if Thursday's ruling applies outside of Utah. A State Department official told Fox News that the agency is reviewing the ruling along with the Justice Department.
“The takeaway from the ruling is that people born in American Samoa living in Utah are now U.S. citizens, and they have all the same rights as other Americans, including the right to vote,” said Neil Weare, president of Equally American and one of the attorneys representing the plaintiffs. “These individuals can now go and register to vote and participate in state, federal and local elections.”
American Samoans can apply for U.S. citizenship but have to pay the $725 application fee, in addition to any legal fees they incur to help them navigate the process.
Fitisemanu said his employment prospects have been diminished because of his rejection from jobs that specify U.S. citizenship as a requirement. In an interview with The Associated Press last year, he said he avoided political conversations because he couldn't vote.
After the ruling, Fitisemanu said he plans to register to vote. The American Samoan government claims automatic U.S. citizenship would undermine local traditions and practices.
The Associated Press contributed to this report.