Michelle Obama said she'll always cherish the moment she slipped into her inaugural ball gown, a one-shouldered, white chiffon design she wore for her first Cinderella-like spins on the dance floor as the US first lady.

She will have to hold on to the memories, because the gown is not exactly hers anymore.

In keeping with tradition, Mrs Obama donated the Jason Wu design to the Smithsonian Institution's National Museum of American History.

It will go on public display in a new gallery for the museum's first ladies exhibit, featuring Presidents' wives from Mamie Eisenhower to the present.

Dotted with beads and covered in fabric petals, Mrs Obama's gown was a star attraction at 10 inaugural balls held on a frigid January 20, 2009 to celebrate Barack Obama's swearing-in earlier that day as the nation's 44th President.

Mrs Obama said memories came rushing back after she saw the gown for the first time since taking it off after that whirlwind night more than a year ago - memories of the freezing cold weather, how hectic the day was and how she had less than an hour to get ready for the balls.

"But I'll never forget the moment that I slipped on this beautiful gown," said Mrs Obama, who wore clothes by another up-and-coming designer to the event - a black-and-white silk twill, off-the-shoulder dress by Prabal Gurung of New York.

"I remember how just luscious I felt as the President and I were announced onto the stage for the first of many dances," she said. "And I'll cherish that moment for the rest of my life."

All first ladies since 1912 have contributed to the collection, though not all first ladies part with their inaugural gowns. Mrs Obama's donation has added significance since hers is the first gown worn by a first lady of colour. The new gallery will feature a total of 10 inaugural dresses.

Mr Wu joined Mrs Obama for the unveiling. He was meeting her for the first time.

"I felt like I've known her for years," he said. "Dressing a woman is one of the ultimate connections."