Spade, a handbag and fashion designer, was found dead in her New York home.
Police said they were investigating her death, at 55, as an apparent suicide.
Spade’s family said in a statement that they were “devastated by Tuesday’s tragedy” and would “miss her terribly”. They asked for privacy.
The designer is survived by her husband, Andy Spade, and their teenage daughter, Frances Beatrix.
Spade, a former editor of fashion magazine Mademoiselle, created a fashion sensation in the 1990s with her line of handbags, and built an accessories empire that grew to hundreds of shops internationally.
“Kate Spade had an enviable gift for understanding exactly what women the world over wanted to carry,” Anna Wintour, the editor-in-chief of Vogue, said in a statement.
Police said the designer’s housekeeper found her unresponsive on Tuesday at her Park Avenue apartment in Manhattan.
The New York police department chief of detectives, Dermot Shea, said a note had been found at the scene but he declined to reveal its contents. He said the evidence at her home pointed to “a tragic suicide”.
The New York Police Department confirmed Spade’s death under her birth name, Katherine Noel Brosnahan.
Spade founded the Kate Spade Handbags brand in 1993 with her partner Andy, whom she had met while studying journalism at Arizona State University, and another friend, Elyce Arons.
The brand opened its first store in New York in 1996, with a stated aim of designing the perfect handbag. Its logo was the spades playing card symbol, and its bright and colourful patterned designs became a hallmark of its products.
She married Andy Spade in 1994 and they had one daughter.
In 1999, Neiman Marcus took a majority stake, sealing the firm’s ascendance in the fashion world and as a brand that embodied quirky fun, without straying too far from the country club.
Celebrities and public figures from the world of fashion and beyond paid tribute to Spade and her work.
Actor David Spade posted a tribute to his sister-in-law, saying she “could make me laugh so hard”.
Actress Reese Witherspoon said the designer was a “wonderful, talented woman who will be dearly missed”.