William Gibbs McAdoo Jr. (October 31, 1863 – February 1, 1941) was an American lawyer and statesman. McAdoo was a leader of the Progressive movement and played a major role in the administration of his father-in-law President Woodrow Wilson. A member of the Democratic Party, he also represented California in the United States Senate.

Born in Marietta, Georgia, McAdoo moved to Knoxville, Tennessee, in his youth and graduated from the University of Tennessee. He established a legal practice in Chattanooga, Tennessee, before moving to New York City in 1892. He gained fame as the president of the Hudson and Manhattan Railroad Company and served as the vice chairman of the Democratic National Committee. McAdoo worked on Wilson's successful 1912 presidential campaign and served as the United States Secretary of the Treasury from 1913 to 1918. He married Wilson's daughter, Eleanor, in 1914. McAdoo presided over the establishment of the Federal Reserve System and helped prevent an economic crisis after the outbreak of World War I. After the U.S. entered the war, McAdoo also served as the Director General of Railroads. McAdoo left Wilson's Cabinet in 1919, co-founding the law firm of McAdoo, Cotton & Franklin.

McAdoo sought the Democratic presidential nomination at the 1920 Democratic National Convention but was opposed by his father-in-law, President Woodrow Wilson, who hoped to be nominated for a third term. In 1922, McAdoo left his law firm and moved to California. He sought the Democratic presidential nomination again in 1924, but the 1924 Democratic National Convention nominated John W. Davis. He was elected to the Senate in 1932 but was defeated in his bid for a second term. McAdoo died of a heart attack in 1941 while traveling from the third inauguration of Franklin D. Roosevelt.