Ruth Lilly (August 2, 1915 – December 30, 2009) was an American philanthropist. She was the daughter of Josiah K. Lilly, Jr., and Ruth (nee Brinkmeyer) Lilly, and the sole heiress to the Eli Lilly and Company pharmaceutical fortune built by her great grandfather, Colonel Eli Lilly.

Lilly made headlines in November 2002 when she pledged stock worth $100 million to the Poetry Foundation, a tiny Chicago nonprofit organization that publishes Poetry Magazine, and a similarly large gift to Americans for the Arts in Washington. The Ruth Lilly Poetry Prize of $100,000 has been awarded annually since 1986 by the Poetry Foundation to honor a living U.S. poet whose lifetime accomplishments warrant extraordinary recognition. The Ruth Lilly Poetry Fellowship of $15,000 has been awarded to young poets each year since 1989.

Giving directly instead of through a foundation, Lilly also supported health care, health education, youth programs, and historic preservation. Indianapolis' Ruth Lilly Hospice, Ruth Lilly Special Collections and Archives, Ruth Lilly Medical Library, Ruth Lilly Health Education Center, Ruth Lilly Law Library, and Ruth Lilly YMCA Outdoor Center all bear her name. In April 2011, Indiana University received two generous gifts from her estate – an estimated $10.7 million—that includes approximately $8 million for the Center on Philanthropy and approximately $2.7 million for the Herron School of Art and Design. With Mrs. Lilly’s gift, the Center will establish the Ruth Lilly Professorship Program, named in her honor, which will provide matching funds to inspire and encourage other donors to create 7 to 10 endowed faculty chairs. Herron will honor Mrs. Lilly by naming its administrative offices the Ruth Lilly Dean’s Suite.