Sybil Mary Hathaway (13 January 1884 - 14 July 1974) was Dame of Sark from 1927 to 1940 and again from 1945 to 1974 (Dame is the title of a female holder of a Seigneurie).

She inherited title when her father died on 14 June 1927, and acquired the name Hathaway upon her second marriage in 1929. Her tenure as Seigneur was interrupted by German occupation of the island during World War II, from 3 July 1940 until 11 May 1945. Hathaway is noted for not evacuating during the German occupation, and prevailing upon all 471 Sark inhabitants to remain on the island as well.

Hathaway commissioned the design of the flag of Sark in 1938. It was also her decision that no cars be allowed on Sark, a rule that continues to the present.

She is the subject of a play The Dame of Sark by William Douglas-Home based on the experience of the German occupation. Upon her death the Seigneurship passed to her grandson, John Michael Beaumont.