The 1918 United States Senate election in Kansas was held on November 5, 1918. Incumbent Democratic Senator William H. Thompson ran for re-election to a second term in the first popular election for this seat after the ratification of the Seventeenth Amendment. Thompson was challenged by Governor Arthur Capper, the Republican nominee. Amid a nationwide Republican wave, Thompson lost to Capper in a landslide, winning just 34% of the vote to Capper's 64%.