Marella Agnelli, a descendant of Neapolitan nobility who lived a rarefied life of palatial estates, ornamental gardens, fine art, high fashion and lofty society, died on Saturday at her home in Turin, Italy. She was 91.

The death was announced by the Agnelli family in a statement.

With her husband, Giovanni Agnelli of the Fiat car manufacturing empire, Mrs. Agnelli owned more than a dozen homes, including an estate in Turin, a ski lodge in the Italian Alps and a Park Avenue apartment in New York. She cultivated an esteemed social circle that included the Kennedys, Henry Kissinger, Katharine Graham and Truman Capote. The couple’s closets were filled with immaculate clothes.

Mrs. Agnelli’s beauty captivated the imagination of artists and fashion designers. Andy Warhol and Richard Avedon created portraits of her. She was among Valentino Garavani’s earliest and most beloved customers. Mr. Capote counted her as one of his “Swans,” the stylish and wealthy women with whom he surrounded himself. He reportedly told Ms. Graham, the Washington Post publisher at the time, that if Mrs. Agnelli and Babe Paley, another of his socialite muses, “were both in Tiffany’s window, Marella would be a little more expensive.”

Mrs. Agnelli remained active and creative in her later years, purchasing and restoring a dilapidated villa in Marrakesh, Morocco, and publishing an autobiographical coffee-table book.