William Hale Thompson Jr. (May 14, 1869 – March 19, 1944) was a corrupt American politician who served as mayor of Chicago from 1915 to 1923 and again from 1927 to 1931. Known as "Big Bill", he is the most recent Republican to have served as mayor of Chicago. Historians rank him among the most unethical mayors in American history, mainly for his open alliance with Al Capone. However, others recognize the effectiveness of his political methods and publicity-oriented campaigning, acknowledging him as a "Political Chameleon" and the leader of an effective political machine.

Thompson was known for his over-the-top campaigning and uncensored language that, along with his towering height and weight, earned him the nickname "Big Bill". Upon his reelection in 1927, Thompson had the school board suspend the Superintendent of Chicago Public Schools, William McAndrew. He was also at the forefront of the movement for the Chicago Public Library system and education officials to censor and ban many texts and historical recollections coming from the United Kingdom.

Though Thompson was a popular figure during his career, his popularity collapsed after his death, when two safe-deposit boxes were found in his name containing over $1.8 million, which were taken as evidence of his corruption.

Thompson was born in Boston, Massachusetts, to William Hale and Medora Gale Thompson, but his family moved to Chicago when he was nine days old. Despite having been born in Boston, Thompson had strong roots in Chicago. His father, Colonel William Hale Thompson Sr., was a popular businessman within Chicago and had served as colonel in the Second Illinois Guard who had come to Chicago after serving in the United States Navy during the American Civil War. His maternal grandfather, Stephen F. Gale, the first chief of the Chicago Fire Department, had played a large part in drawing up the city's corporation charter in 1837, earning him regard as a "Chicago pioneer" by some academic journalists.

Thompson's father, William Hale Thompson Sr., served as the 24th ward member of the Cook County Republican Party Central Committee.

Thompson attended Chicago Public Schools, and took supplementary courses at the Fessenden School and Metropolitan College.

Thompson was meant to attend Yale but instead moved to Wyoming at the age of 14, where he became a cowboy and cattle owner and traveled across Europe, taking up ranching in Texas and New Mexico later on in his life. The experiences influenced him to add Western touches into his campaign, including his sombrero, which became a symbol for his campaign. By the age of twenty-one, he had accumulated a stake of $30,000. He returned to Chicago in 1892 after his father's death to manage his estates. Shortly after returning to Chicago, Thompson joined the Illinois Athletic Club and the Sportsmen's Club of America and quickly was appointed director-general and captain of the Chicago Athletic Association's water polo and football teams. His six-foot frame and athletic prowess earned him the nickname "Big Bill," which stuck with him throughout his career as a politician.

Thompson served on the Real Estate Board of Chicago.