Jenny Marx Longuet (Jennychen) (11 May 1844 – 11 January 1883) was the eldest daughter of Jenny von Westphalen and Karl Marx.
Known as 'Jennychen' in the Marx circle, Jenny Longuet, Marx's eldest daughter, was a socialist activist. She wrote for the socialist press in France in the 1860s, most importantly in exposing British treatment of Fenian revolutionaries in Ireland.
She married Paris Commune veteran Charles Longuet on 2 October 1872. She was the mother of five boys and a daughter.
Jenny Longuet died at Argenteuil near Paris on January 11, 1883, at the age of 38, probably from cancer of the bladder, a condition which had afflicted her for some time.