Joseph Arnold Weydemeyer (February 2, 1818, Münster – August 26, 1866, St. Louis, Missouri) was a military officer in the Kingdom of Prussia and the United States, as well as a journalist, politician and Marxist revolutionary.
At first a supporter of "true socialism" he became, in 1845-46, a follower of Karl Marx and Friedrich Engels. He became a member of the League of Communists. From 1849 to 1851 he headed its Frankfurt chapter. He visited Marx in Brussels, staying there for a time to attend Marx’s lectures. He participated in the 1848 Revolution. He was one of the "responsible editors" of the Neue Rheinische Zeitung from 1849 to 1850. He was involved in the writing of the manuscript of the Deutsche Ideologie (German Ideology).
He worked on two socialist periodicals which were the Westphälisches Dampfboot (Westphalian Steamboat) and the Neue Rheinische Zeitung. In 1851 he emigrated from Germany to the United States and worked there as a journalist. The Eighteenth Brumaire of Louis Napoleon, written by Karl Marx, was published in 1852 in "Die Revolution", a German-language monthly magazine in New York, established by Weydemeyer.
Weydemeyer took part in the US Civil War as a lieutenant colonel in the Union Army.
Born in 1818, the same year as Karl Marx, Weydemeyer was the son of a Prussian civil servant residing in Münster in Westphalia. Sent to a gymnasium and the Berlin military Academy, he received his commission as a Leutnant in the Prussian artillery (1. Westfälisches Feldartillerie-Regiment Nr. 7) in 1838. At the beginning of his short career, he was stationed in the Westphalian town of Minden. He began to read the bourgeois radical and socialist newspaper Rheinische Zeitung, the Cologne paper Marx became editor and which was suppressed by Prussian censorship in 1843. But it inspired many soldiers in the Rhineland and Westphalia. In the Minden garrison, the paper inspired revolutionaries like Fritz Anneke, August Willich, Hermann Korff and Friedrich von Beust, all of whom, like Weydemeyer, will become prominent Forty-Eighters and after that officers of the Union army in the Civil War.