Charles Félix César Longuet (14 February 1839, Caen – 5 August 1903, Paris) was a journalist and prominent figure in the French working-class movement, including the 1871 Paris Commune, as well as a Proudhonist member of the General Council of the First International or International Working Men's Association (1866–67, 1871–72). He served as Corresponding Secretary for Belgium (1866),^[1] delegate to the Lausanne^[2] (1867), Brussels (1868), the London Conference (1871) and the (1872).^[3] He was also the editor of the publication Journal Officiel.^[4]

Longuet participated in the Paris Commune of 1871 and, after its defeat, moved to England as a refugee where he met Karl Marx. Longuet married Marx's eldest daughter, Jenny, on 2 October 1872 in London (in a civil ceremony). Together, they had six children, the first five of whom were boys, the last a daughter.^[5] Two of the sons died in infancy. Of the others, Jean, a journalist and Edgar, a physician, both became prominent socialist activists in France.^[6]

Longuet returned to France, after a political amnesty granted by the French government in July 1880. Here he took a position as an editor of La Justice, a radical daily newspaper founded by Georges Clemenceau.^[7] His wife and children joined him in February 1881, the family settling in the town of Argenteuil, near Paris.^[8] Here Jenny died in January 1883, probably from cancer of the bladder.^[5] Two months later her father, Karl Marx, died; Longuet was one of the speakers at his funeral.^[9]

Charles Longuet died in Paris on 5 August 1903 at the age of 64. He was buried at the Père Lachaise Cemetery.