Kathryn Day (née Bouleyn) is an American opera singer who has had an active international career spanning five decades. She began her career as a leading soprano under the name Kathryn Bouleyn in the 1970s and 1980s with companies like the New York City Opera, the San Francisco Opera, and the Opera Theater of Saint Louis. With the latter institution she created the role of Cora in the world premiere of Stephen Paulus' The Postman Always Rings Twice (1982).