The beloved designer, famed for her brand's exuberant colour palettes and fun prints, was found by her housekeeper in her Park Avenue apartment on Tuesday, the ABC reports.
Born Katherine Noel Brosnahan, Spade launched her eponymous label with her husband, Andy Spade, in 1993. They sold the brand in 2006.
Kate Spade New York has released a statement saying their founder "will be dearly missed."
The New York Times reports that Spade had left a note, but police have not released its contents.
Her family have also released a statement, writing, “We loved Kate dearly and will miss her terribly. We would ask that our privacy be respected as we grieve during this very difficult time.”
Spade is survived by her husband and their 13-year-old daughter, Frances Beatrix Spade.