William J. Duff (August 17, 1856 – May 26, 1922) was an American football player, medical doctor, and state legislator.

Duff was born in Allegheny City, Pennsylvania, in 1856.

In 1868, the family moved to Fort Gratiot, now a part of Port Huron, Michigan.

His father, George Duff, was an officer in the U.S. infantry stationed at Fort Gratiot.

[1][2] In 1885, Duff returned to Port Huron and entered the practice of medicine.