Karl Deichmann (5 October 1863 – 12 February 1940) was a German trade unionist and politician.
He moved to Bremen in 1884, and soon joined the Social Democratic Party of Germany (SPD) and the German Tobacco Workers' Union.
He became associated with the right wing of the trade union movement, and worked closely with Friedrich Ebert.
In 1900, he was elected as the president of the union, and from 1910 until 1918, he additionally served as General Secretary of the International Federation of Tobacco Workers.
He was then elected to the regularly constituted Bürgerschaft, which appointed him to the Senate of Bremen.