Mary Hayward Weir (1915-1968) was an American steel heiress.

Mary Hayward Weir was the wealthy widow of Pittsburgh steel king Ernest T. Weir , who died at the age of 81 in 1957. The couple had been married since December 1941 and together had one son, David Weir, born in 1944. In January 1962 she married the Polish author Jerzy Kosinski after a one year and a half long romance. Kosinski had emigrated from Poland in December 1957 to the United States. Hayward Weir and Kosinski met in the summer of 1960 in New York City, when the heiress hired the young Kosinski to catalogue her private library. The two soon fell in love with each other. Hayward Weir was also a friend of coffee heiress Abigail Folger of the Folgers Coffee Company family.

Miss Hayward Weir was known for both her never ending volunteer work in many different areas, as well as her extensive travels around the world. A public library was named in her honor in 1958 with Hayward Weir cutting the ribbon in person herself.

In 1965, her husband dedicated his novel "The Painted Bird" to her and it was at this time that she began to feel ill. Later in 1966, Hayward Weir divorced Kosinski, but she still remained a close friend. Mary was later diagnosed with a brain tumour, of which she died on August 1,1968.

Rumors still persist that Hayward Weir may have committed suicide.