The woman, a resident of Jefferson County in western Pennsylvania, died Saturday from complications of a probable measles case.
County Coroner Greg Furlong confirmed the death on Sunday.
She had not been vaccinated, and hers is the first adult death in the current US outbreak.
All three of this year's US measles deaths have been in Pennsylvania.
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The two earlier deaths were in Lancaster County in late August. One was an infant, and both were unvaccinated.
Before those cases, no one had died of measles in Pennsylvania for 35 years.
"This is a heartbreaking loss for the family and an unfortunate reminder that measles can be a serious and potentially life-threatening disease," Furlong said.
The worst US measles year in decades
The CDC has counted 3,294 confirmed measles cases nationally as of September 10, spread across 47 jurisdictions and tied to 38 active outbreaks.
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It is the country's largest annual case count since 1991.
About 95 percent of confirmed cases involve people who are unvaccinated or whose vaccination status is unknown.
Pennsylvania alone has confirmed 676 cases in 37 counties this year, with 124 hospitalizations.
Fewer than 1 percent of the state's confirmed cases were in fully vaccinated people, according to the state Department of Health.
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The Lancaster County outbreak has been the center of the year's cluster.
It is a county where MMR vaccination coverage among children slid from around 95.5 percent in 2017 to 87.6 percent last year.
What the CDC recommends for adults
The CDC says two doses of the MMR vaccine are 97 percent effective at preventing measles, and one dose is 93 percent effective.
Under CDC guidance, "All adults who do not have evidence of immunity should be protected against measles, mumps, and rubella with at least 1 dose of MMR vaccine."
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A second dose is recommended for higher-risk adults, including healthcare workers, international travelers, post-secondary students and household contacts of immunocompromised people.
The Pennsylvania Department of Health said it is reviewing the Jefferson County case and will release additional information as its investigation progresses.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.