Michigan Democrat Rashida Tlaib has been sworn into office while wearing a traditional garment stitched by her Palestinian-born mother.
Ms Tlaib had been expected to take her oath on a Koran owned by Thomas Jefferson, but changed her mind, according to the Detroit Free Press.
Ms Tlaib and Minnesota's Ilhan Omar became the first-ever Muslim female members of Congress on Thursday.
Native-American Deb Haaland also wore traditional dress for the ceremony.
The New Mexico Democrat, who is one of two first female Native American members of Congress, wore traditional dress of the Pueblo tribe, including silver and turquoise jewellery and moccasins.
On Thursday, hundreds of lawmakers from across the US arrived in Washington to be sworn in as members of Congress.
Although no text is required for the oath, many US lawmakers have traditionally held the Bible as they vowed to protect the US Constitution from "all enemies, foreign and domestic".
In an interview with the Detroit Free Press, Ms Tlaib, 42, said she would be using the Koran owned by America's third president, but she later told the paper that she had instead decided to use a family Koran.
"My swearing in on the Koran is about me showing that the American people are made up of diverse backgrounds and we all have love of justice and freedom".
"It's important to me because a lot of Americans have this kind of feeling that Islam is somehow foreign to American history," she said.
"Muslims were there at the beginning... Some of our founding fathers knew more about Islam than some members of Congress now."
Ms Tlaib, a Democrat representing Detroit, told the Free Press that she chose to wear a traditional Palestinian garment, called a thobe, which was made by her mother who came to the US from the West Bank when she was 20 years old.
Ms Tlaib is a mother of two boys and is the oldest of her 14 siblings.
Her grandmother still lives in the West Bank and she has said that she hopes to lead a US delegation there after beginning her term.
On social media, she shared images of the dress and asked others to #TweetYourThobe, inspiring followers to share pictures of their dresses.
While celebrating the Democrats' newest members, some on Twitter pointed out a lack of diversity among Republicans.
Watching the 116th Congress mill around from up in the press gallery, the visual difference between the Republican side and the Democratic side is noticeable. On one side, mostly white men in suits. On the other, more women wearing all colors and more diversity of skin tone.— Caroline Simon (@carolinesimon66) January 3, 2019