Jean-Laurent-Frederick Longuet (1876–1938) was a French socialist politician and journalist. He was Karl Marx's grandson.
Jean, often called 'Johnny' as a boy by his family, was born in London on 10 May 1876, the son of Charles and Jenny Longuet. He was their second son, and the eldest who survived to adulthood. The family often visited Jenny's father, Karl Marx, who liked to play with his grandchildren.
The Longuet family moved to France in February 1881. In summer 1882 Karl Marx stayed with the Longuets for three months, being joined by Jean's aunt Eleanor Marx. By this time Jenny was suffering from bladder cancer, and would die a year later. To ease the burden on the family, Eleanor took Jean back to England in August 1882, promising to educate and discipline him. They became close, with Eleanor thinking of him as ‘my boy’. On his return to France, Jean lived for a time with his father's family in Caen to continue his studies.
After attending university in Paris, Longuet worked as a journalist and trained as a lawyer. He worked for L'Humanité and was a founder and editor of the newspaper Le Populaire. He was active in one of France's principal socialist parties – the French Section of the Workers' International (SFIO) – and served both as a mayor and as a member of the French Chamber of Deputies.
During the First World War he was a pacifist, but also supported war credits. At the Strasbourg Congress in 1918 his policy was adopted by the majority of the SFIO socialist party. After the Tours Congress of 1920 where the Communists gained the majority, he supported the minority and joined the centrist Two-and-a-half International (the Vienna Union). He criticized the League Against Imperialism created in 1927 and supported by the Comintern.
Jean Longuet married Anita Desvaux (1875–1960) in 1900. They had two sons: the lawyer and journalist Robert-Jean Longuet (1901–1987) and the sculptor Karl-Jean Longuet (1904–1981). Jean's younger brother Edgar Longuet, a physician, was also an active socialist.
Longuet died after a car accident in September 1938 aged 61. He was buried at the Père Lachaise Cemetery in Paris, in the same grave as his aunt and uncle, Laura and Paul Lafargue. Longuet's wife and two sons were later buried in the same grave.