It’s a whale of a mistake!

Shoppers yesterday slammed Target for labeling the same dress “manatee gray” in plus sizes and “dark heather gray” in smaller sizes — claiming the plus-sized label evokes a cruel fat-girl joke.

“It is insulting — they should have known better,” said Claudia Belisle, 29, shopping at the Atlantic Terminal Mall Target in Brooklyn.

“It is putting down one set of people over another,” said Michelle Ho, 26, another shopper.

A Target spokeswoman, Jessica Deede said the labeling of the Massimo-brand “Women’s Plus-Size Kimono Maxi Dress” was “an unintentional oversight and we apologize.”

Although manatee gray was a seasonal color, Deede said, Target is “fixing the discrepancy” and the product was removed from the company Web site.