Sister Mary Luke Tobin SL (May 16, 1908 – August 24, 2006) was an American Roman Catholic religious sister, and one of only 15 women auditors invited to the Second Vatican Council, and the only American woman of the three women religious permitted to participate on the Council's planning commissions.

[1] Christened as Ruth Marie Tobin, she attended public schools in Denver and traveled to Nevada and California with her parents and older brother.

She did much of her work in her native Denver but traveled the world on missions for peace, including visits to Saigon, Paris, El Salvador and Northern Ireland.

While living at the Loretto Motherhouse in Nerinx, Kentucky, she became friends with Trappist monk Thomas Merton.

[2] The diminutive Sister supported women's ordination to the priesthood, opposed nuclear proliferation, supported the United Farm Workers and took on the Blue Diamond Coal Company by using Loretto's shares to challenge the firm's practices and took part in nonviolent actions at Rocky Flats Nuclear Weapons Plant, the U.S. Air Force Academy and Martin-Marietta in Colorado.