Johann Friedrich "Fritz" Westmeyer (14 January 1873 – 14 November 1917) was a German trade unionist and socialist politician.

Educated at Osnabrück schools, Westmeyer worked as a chimney sweep and became a trade unionist and member of the Social Democratic Party of Germany (SPD) in the 1890s.

Westmeyer spent the subsequent years as an activist and organizer in Stuttgart, where he became a close ally of Clara Zetkin, an influential Marxist who would become a leading member of the Spartacist League, the Independent Social Democratic Party of Germany (USPD), and the Communist Party of Germany (KPD) after the collapse of the German Empire in the First World War.

He ran as a candidate for the Reichstag in 1907, was elected to lead the Social Democratic Association in Stuttgart in 1908-1914, and edited the Schwäbische Tagwacht.

Arrested by the authorities along with other significant Württemberg SPD radicals, Westmeyer was forcibly mobilized into imperial Germany's wartime army and sent to carry out his service on the Western Front despite his age and opposition to the war.