William A. Cassidy is an American geologist and professor emeritus of Geology and Planetary Science at the University of Pittsburgh. Cassidy was responsible for recognizing that Antarctica represented the greatest repository of meteorites on earth. Starting in the early 1970s, Cassidy led most major meteorite expeditions to the south polar region, and in 1979, he was awarded the Antarctica Service Medal.
The Cassidy Glacier in Antarctica is named for him, and the mineral Cassidyite is named in his honor.
- Meteorites, Ice, and Antarctica: A Personal Account (2003, Cambridge University Press, ISBN 978-0521258722)