William Mills "Bill" Irwin (born April 11, 1950) is an American actor and clown noted for his contribution to the renaissance of American circus during the 1970s. He is known for his vaudeville-style stage acts, but has made a number of appearances on film and television and won a Tony Award for a dramatic role on Broadway.

Irwin was born in Santa Monica, California, the son of Elizabeth (née Mills), a teacher, and Horace G. Irwin, an aerospace engineer. He graduated from Oberlin College in 1973 with a degree in theater arts, and from the Ringling Brothers and Barnum & Bailey Clown College the following year. In 1975, he helped found the Pickle Family Circus in San Francisco, California. He left the company in 1979, and decided to pursue stage work.

Irwin created a run of highly regarded stage shows that incorporated elements of clowning, often in collaboration with composer Doug Skinner. These works included The Regard of Flight (1982), Largely New York (1989), Fool Moon (1993), The Harlequin Studies (2003), and Mr. Fox: A Rumination (2004). Mr. Fox is a production that Irwin has worked on for years, a biography of 19th century clown George Washington Lafayette Fox that also has autobiographical elements.