William L. Fash, Jr. (born 1954) is an American anthropologist who is the Bowditch Professor of Central American and Mexican Archaeology and Ethnology at Harvard. In 2008, he was given the Hoja de Laurel de Oro by the Government of Honduras for 30-plus years of "preserving and documenting Honduras' cultural heritage." He is corresponding fellow of the Academia Mexicana de la Historia since March 2017.

In 1975, Fash excavated at Grasshopper, Cibecue, Arizona as a student in a summer field school. He received his B.A. in anthropology from the University of Illinois Urbana-Champaign in 1976. That same year he received a National Science Foundation Graduate Fellowship for the research he would do at Harvard, from which he graduated with a Ph.D. in 1983.

Besides his participation at the Grasshopper Site, Fash has participated in many excavations at Copan, some of those span several years and involve multiple researchers. Often, he was the director of these excavations. He first worked in Chalcatzingo, Mexico in 1974 and again at the same site in 1976, it would be 24 years before he worked at a site in Mexico again. Below are some of the excavations he participated in:

Proyecto Arqueologico Copan – Hw had four different positions during this project over seven years. He worked as an archaeologist, Director of Settlement Surveys and Excavations, Excavations Director, and Research and Museums Consultant.

Copan Temples Project – He began work on this project the same year as the Copan Mosaics Project, it functioned as an international field school. He directed it for four years (1985–1990).

Copan Mosaics Project (1985–present) – He initiated (with Barbara Fash and Rudy Larios) and directs this project, which started in 1985 but is currently unfinished. In 1987, Fash was awarded a Fulbright Senior Research Fellowship for this project, whose purpose is "to record and protect Copan’s hundreds of stone mosaic sculptures." To give an idea of the extent of the project, involves all of Copan's Principle group and preservation of its Hieroglyphic Stairway.

Copan Acropolis Archaeological Project (1988–1997) – He directed this project for 8 years (1988–1996), it involved the excavation of temples, courts, and tombs at Copan. Like the Copan Mosaics Project, this project also aided the architectural sculpture cataloguing process. Other researchers that assisted this project include Robert Sharer and Ricardo Agurcia Fasquelle.

Harvard Field School in Maya Archaeology at Copan, Honduras – He directed this project for 6 years.