William Willard Wirtz, Sr. was an American football, basketball, and baseball coach. He served as the head football coach at Northern Illinois University from 1910 to 1916, compiling a record of 33–17–9. Wirtz was also the head basketball coach at Northern Illinois from 1910 to 1918 and the head baseball coach at the school from 1912 to 1917. He was the father of W. Willard Wirtz.