William Jamison Gavigan (September 17, 1859 – February 18, 1925) was an American printer, attorney, physician and politician who served in the California State Assembly from 1881 to 1883. He was elected on a Workingmen's-Democratic Fusion ticket at just 21 years, one month and 15 days old, making him the youngest person ever elected to the California State Legislature. As of 1926, Gavigan was distinguished as one of only two politicians in California state history to be both a medical doctor and an attorney-at-law, the other being Edward Robeson Taylor.