Hamilton Fish III (born Hamilton Stuyvesant Fish and also known as Hamilton Fish Jr.; December 7, 1888 – January 18, 1991) was an American soldier, author, and politician from New York. He represented New York's 26th congressional district in the Hudson Valley region in the United States House of Representatives from 1920 to 1945. In the second half of his House career, Fish was a chief critic and opponent of President Franklin D. Roosevelt, especially on matters of international affairs and American entry into World War II prior to the attack on Pearl Harbor.

Born into a political family whose legacy dated to the Dutch settlement of New Amsterdam, Fish was educated at St. Mark's School and Harvard College, where he graduated at the age of 20 before enrolling at Harvard Law School. He was elected to the New York State Assembly from Putnam County in 1913 as a member of the Progressive Party. He served three terms before enlisting in World War I, in which he commanded a company of the 369th Infantry Regiment, a unit of African-American soldiers known as the "Harlem Hellfighters." After the war, Fish was elected to the United States House as a Republican. In the House, he advocated for veterans and the anti-lynching movement. He became the body's leading anti-communist as chair of the Fish Committee, a special body established in 1930 to investigate Soviet and communist influence in the United States. He also sponsored the Lodge–Fish Resolution, expressing American support for the British establishment of a Jewish state in Palestine.

In 1932, Fish crossed party lines to privately support Hudson Valley native Franklin D. Roosevelt for president, but he soon became a leading critic of Roosevelt's New Deal legislation and Atlanticist foreign policy. Throughout the 1930s, Fish was the subject of multiple foreign influence campaigns, since he was identified by Nazi Party officials as a natural ally to their international ambitions (though he was on the record criticizing the treatment of Jews in Germany) and by British security organizations as an obstacle to American aid for Great Britain. His chief of staff, George Hill, was convicted of perjury in relation to his involvement with George Sylvester Viereck, and agents of the British Security Co-ordination office repeatedly raised money for Fish's opponents and published anti-Fish leaflets in his district. In 1941, Fish was implicated in an America First Committee franking controversy leading to William Power Maloney's grand jury investigating Nazi penetration in the United States.

After the 1941 Japanese attack on Pearl Harbor and German declaration of war, Fish called for unified support for Roosevelt as a wartime president. He delivered the first speech calling for a declaration of war against Japan and sought to re-enlist but was denied a commission by Roosevelt; he later returned to his criticism, arguing that Roosevelt should have prevented the Japanese strike. Like many leading isolationists, Fish's popularity waned during wartime, and he was defeated for re-election in 1944. Late in the decade, former United States Department of Justice prosecutor O. John Rogge accused Fish of Nazi sympathies. He continued to actively comment on American diplomacy and military strategy during and after the Cold War, criticizing the United Nations, John Foster Dulles, and the Vietnam War while supporting NATO, the American invasions of Grenada and Panama, and the Gulf War.

Hamilton Stuyvesant Fish was born on December 7, 1888, in Garrison, New York to Hamilton Fish II and the former Emily Mann. Hamilton Fish II had been a Republican member of the New York Assembly and would later serve as Speaker of the Assembly and a United States Representative. Hamilton Fish, the paternal grandfather of this article's subject, was United States Secretary of State under President Ulysses S. Grant.

In 1898, the young Fish's cousin, also named Hamilton Fish and a volunteer in Theodore Roosevelt's Rough Riders, was the first American soldier killed in action in the Spanish–American War. In his honor, Fish's father legally changed his ten-year-old son's name from Hamilton Stuyvesant Fish to just Hamilton Fish.

During his childhood, Fish attended Chateau de Lancy near Geneva, which his father had also attended. The younger Fish learned French, played soccer, and spent summers with his family in Bavaria. He returned to the United States to attend the Fay School in Southborough, Massachusetts and St. Mark's School, a preparatory school also in Southborough. In his memoirs, Fish described himself as a "B student" at St. Mark's but successful in several different sports.

Fish graduated from St. Mark's in 1906 and attended Harvard College, where he graduated in the class of 1910. He was a member of the Porcellian Club and played tackle for Harvard's football team. Standing 6 feet 4 inches (1.93 m) and weighing 200 pounds (91 kg), "Ham" Fish was highly successful as a football player; he was twice an All-America and was inducted into the College Football Hall of Fame in 1954. He was the only Harvard man on Yale graduate Walter Camp's all-time All-American team. After graduating, Fish continued his involvement in football. He donated $5,000.00 for several awards to Harvard football players and organized the Harvard Law School football team, which played exhibition games with other colleges around the country. During his time at Harvard, Fish became friends with Theodore Roosevelt Jr., the son of President Theodore Roosevelt.