Sandy Dennis (April 27, 1937 – March 2, 1992) was an Academy Award-winning American theater and film actress.

Born Sandra Dale Dennis in Hastings, Nebraska, and a high-school classmate of Dick Cavett, Dennis made her television debut in 1956 in The Guiding Light and her film debut in Splendor in the Grass (1961). However she was more committed to following a career in the theater.

She won consecutive Tony Awards for her performances in A Thousand Clowns and Any Wednesday, and the Oscar for Best Supporting Actress for her role in Who's Afraid of Virginia Woolf? (1966). She followed this with well-received performances in Up the Down Staircase (1967), The Fox (1967), Sweet November (1968) and The Out-of-Towners (1970).

An advocate of method acting, Dennis was often described as neurotic and mannered in her performances; her signature style included running words together and oddly stopping and starting sentences, suddenly going up and down octaves as she spoke, and fluttering her hands. Walter Kerr famously remarked that she treated sentences as "weak, injured things" that needed to be slowly helped "across the street." Nonetheless, William Goldman, in his book The Season, referred to her as a quintessential "critics' darling" who got rave reviews no matter how unusual her acting and questionable her choice of material.

Her last significant film role was in Come Back to the Five and Dime, Jimmy Dean, Jimmy Dean (1982). Dennis lived for many years with jazz great Gerry Mulligan until they split up in 1976. Although Mulligan often referred to Dennis as his second wife, Dennis later revealed that they had never married. She also lived with actor Eric Roberts from 1980 to 1985.

She died from ovarian cancer in Westport, Connecticut, aged 54.