Leonidas Carstarphen Dyer (June 11, 1871 – December 15, 1957) was an American politician, reformer, civil rights activist, and military officer. A Republican, he served eleven terms in the U.S. Congress as a U.S. Representative from Missouri from 1911 to 1933. In 1898, enrolling in the U.S. Army as a private, Dyer served notably in the Spanish–American War; and was promoted to colonel at the war's end.

Working as an attorney in St. Louis, Dyer started an anti-usury campaign and was elected to Congress as a Republican in 1910. As a progressive reformer, Dyer authored an anti-usury law in 1914 that limited excessive loan rates by bank lenders in the nation's capital, then still governed by Congress.

Horrified by the East St. Louis riots in 1917 and the high rate of reported lynchings in the South, Dyer introduced the Dyer Anti-Lynching Bill in 1918. In 1920, the Republican Party supported such legislation in its platform from the National Convention. In January 1922, Dyer's bill was passed by the House, which approved it by a wide margin due to "insistent countrywide demand". The bill was defeated by filibusters by white conservative, Southern Democrats in the U.S. Senate in December 1922, in 1923, and 1924.

In 1919, Dyer authored the motor-vehicle theft law, which made transporting stolen automobiles across state lines a federal crime. By 1956, the FBI reported that the law had enabled the recovery of cars worth more than $212 million. In terms of Prohibition, Dyer voted against various anti-liquor laws, including the Eighteenth Amendment to the U.S. Constitution. Dyer served in Congress from the 62nd Congress to the 72nd Congress. He was defeated for re-election in 1932.