It’s a fashion first . . . lady.

Michelle Obama is once again setting the style world abuzz — wearing a thrift-store dress to the “Christmas in Washington” concert, which was taped Sunday and airs on TNT tonight.

The “couture” black-lace overlay frock, featuring a high, square neckline and a full skirt was plucked from New York Vintage in Chelsea.

The secondhand 1950s piece — believed to be the first vintage dress worn by a first lady to a public event — was made by American designer Norman Norelli, who died in 1972 and was among the most influential designers of his time.

The store’s owners would not say how much the first lady paid for the dress, but prices for vintage clothing have risen dramatically as quality pieces have become wildly popular among celebrities and on the red carpet. One estimate for this hand-me-down was $2,500.

Obama’s fashion choices frequently make headlines. In the past, her outfits have immediately become hot sellers at department stores and specialty retailers — even leading to long lines and waiting lists for specific pieces.

Unfortunately for her admirers, this dress is one of a kind.

An owner of New York Vintage said Michelle Obama is expected to wear more vintage clothing in the future.

“We are truly honored that the first lady has chosen a gown from New York Vintage as her first venture into vintage couture,” co-owner Jon Schneck told The Huffington Post. “Other dresses from our archive have been scheduled for use by the first lady later this year.”

Before the concert at the National Building Museum in DC, Obama, who paired the gown with oversize snowflake earrings, took the stage with President Obama, their daughters, Malia and Sasha, her mother, Marian Robinson, and a group of children dressed as elves in front of a Christmas tree.

“Christmas in Washington,” hosted by Ellen DeGeneres and featuring Mariah Carey, Annie Lennox and Miranda Cosgrove, airs tonight at 8 on TNT.

don.kaplan@nypost.com

