Kathleen, Marchioness of Hartington (Kathleen Agnes Cavendish, née Kennedy) (February 20, 1920 – May 13, 1948), was the second daughter of Joseph P. Kennedy, Sr. and a sister of American President John F. Kennedy.

Despite the opposition of her intensely Catholic mother, on May 7. 1944, Kathleen Kennedy, known to friends as "Kick," married William Cavendish, Marquess of Hartington, a Protestant and the eldest son and heir of the 10th Duke of Devonshire. Four months later, he was killed by a German sniper in World War II. (Billy Hartington's younger brother, Andrew, married to Deborah Mitford, became heir to the dukedom.) Lady Hartington's mother, Rose Kennedy, reportedly interpreted her son-in-law's death as retribution from God.

Popular on the London social circuit and admired by many for her high spirits -- more traditional members of British society found fault with her boisterousness, however -- the dashing young widow eventually became the mistress of Peter Wentworth-Fitzwilliam , 8th and last Earl Fitzwilliam. The couple planned to wed after Fitzwilliam's planned divorce. Instead, while on a trip to visit Joseph Kennedy Sr. and gain his blessing for their relationship, both Lord Fitzwilliam and Lady Hartington died in an airplane crash over Sainte-Bauzille , Ardèche, France.

She is buried in the Cavendish family plot at Saint Peter's Church, Edensor Village , Chatsworth, England.

For the late 20th century film producer see, Kathleen Kennedy (movie producer).