William Hamilton Jefferys III (born 1940) is an American astronomer. He is a Harlan J. Smith Centennial Professor of Astronomy (Emeritus) of astronomy at The University of Texas at Austin, and an adjunct professor of statistics at the University of Vermont. Jefferys served as chairman of The University of Texas Department of Astronomy from 1994 to 1998. He retired from the University of Texas in 2004 and moved to Vermont in 2005, where he accepted an appointment as adjunct professor of statistics at the University of Vermont.