A previously unknown photograph of Harriet Tubman, the abolitionist leader known as the “Moses of her people,” is on display for the first time in Washington.

The photograph, on view at the Smithsonian’s National Museum of African American History and Culture, is significant because it shows a young Tubman casually seated in a chair wearing an elegant dress with an elaborate bodice and full skirt. Other surviving photographs of Tubman show her looking stern or pensive and, in her later years, frail and wan.

“What this photograph does is humanize Harriet Tubman,” said Lonnie G. Bunch III, the founding director of the museum. “The photograph shows her stylish and in the vibrancy of her youth.”

Tubman, whose original name was Araminta Ross, was born into slavery in Dorchester County, Md., in the early 1820s. She would later come to be known for her work guiding enslaved people to freedom via the Underground Railroad, a loosely connected network of abolitionists who monitored safe houses where fugitives were sheltered on their journey north to free states.

In 1849, Tubman escaped to Philadelphia. (An offer of a $100 reward for her capture was published in the Cambridge Democrat.) Soon after, she began to make trips back to the South to help others on their trek to freedom. She also became friends with Frederick Douglass, the abolitionist leader.

During the Civil War she worked as a scout and spy for the Union Army, providing intelligence and continuing her work liberating enslaved people. After the war ended in 1865, she settled in Auburn, N.Y., where she remained prominent as a women’s rights activist and died in 1913.

In 2016, then-Treasury Secretary Jacob J. Lew proposed that Tubman replace Andrew Jackson on the $20 bill. The proposal has stalled under the Trump administration.

The photograph of Tubman, which is believed to have been taken in the late 1860s, was acquired by the museum two years ago, part of a leather-bound album owned by Emily Howland, a Quaker schoolteacher who worked in Arlington, Va.

The album was originally compiled as a gift for Ms. Howland and contained 49 images from the 1860s. It included portraits of Lydia Maria Child, an abolitionist and women’s rights activist, and John Willis Menard, the first black man elected to the House of Representatives. Conservators for the Library of Congress cleaned the photographs and repaired the album so it could be displayed.

Mr. Bunch said he and his colleagues had been hearing about the portfolio for years and, when it came up for auction, they joined forces with the Library of Congress to buy it.

“We thought we knew everything we were going to know about Harriet Tubman,” Mr. Bunch said. “This inspires us to keep looking.”