Jenny Julia Eleanor Marx (16 January 1855 – 31 March 1898), sometimes called Eleanor Aveling and known to her family as Tussy, was the English-born youngest daughter of Karl Marx. She was herself a socialist activist who sometimes worked as a literary translator. In March 1898, after discovering that her partner Edward Aveling had secretly married the previous year, she poisoned herself at the age of 43.

Eleanor Marx was born in London on 16 January 1855, the sixth child and fourth daughter of Karl Marx and his wife Jenny von Westphalen. She was called "Tussy" by her family from a young age. She showed an early interest in politics, even writing to political figures during her childhood. The hanging of the "Manchester Martyrs" when she was twelve, for example, horrified her and shaped her lifelong sympathy for the Fenians. Her father's story-telling also inspired an interest in literature, and she could recite passages by William Shakespeare at the age of three. By her teenage years, that love of Shakespeare led to the formation of the "Dogberry Club" at which she, her family, and the family of Clara Collet, all recited Shakespeare whilst her father watched.

While Karl Marx was writing his major work, Das Kapital, in the family home, his youngest daughter Eleanor played in his study. Marx invented and narrated a story for Eleanor based on an anti-hero called Hans Röckle. Eleanor reported that it was one of her favourite childhood stories. The story is significant because it offered Eleanor lessons, by allegory, of the critique of political economy which Marx was writing in Das Kapital. As an adult, Eleanor was involved in translating and editing volumes of Das Kapital. She also edited Marx's lectures, Value, Price and Profit and Wage Labour and Capital, which were based on the same material, into books. Eleanor Marx's biographer, Rachel Holmes, writes: "Tussy's childhood intimacy with [Marx] whilst he wrote the first volume of Das Kapital provided her with a thorough grounding in British economic, political and social history. Tussy and Capital grew up together".

At the age of sixteen, Eleanor became her father's secretary and accompanied him around the world to socialist conferences. A year later, she fell in love with Prosper-Olivier Lissagaray, a journalist and participant in the Paris Commune, who had fled to London after the Commune's suppression. Although he agreed with the man politically, Karl Marx disapproved of the relationship because of the age gap between the two, Lissagaray being 34 years old. In May 1873, Eleanor moved away from home to Brighton working as a schoolteacher. She lived at 6 Vernon Terrace in the suburb of Montpelier, returning to London in September 1873.

In 1876, Eleanor helped Lissagaray write History of the Commune of 1871, and translated it into English. Her father liked the book but was still disapproving of his daughter's relationship with its author. By 1880, Karl changed his view of the situation, and allowed her to marry him. By then, however, Eleanor herself was having second thoughts, and she terminated the relationship in 1882.

In the early 1880s, she nursed her aging parents. Her mother died in December 1881 but, from August 1882, she also cared for her young nephew Jean Longuet for several months, easing the burden on her elder sister, Jenny Longuet, who died in January 1883 of bladder cancer. Her father died two months later, in March 1883. After that, Eleanor and Edward Aveling, overseen by Friedrich Engels, prepared the first English language edition of Das Kapital volume I, published in 1887. On Engels' death in 1895, she and Aveling sorted and stored her father's extensive papers.

Marx identified strongly with her Jewish heritage. In a reversal of her paternal grandparents' abandonment of Judaism and conversion to Christianity, she proudly declared: "I am a Jewess". Her interest in her Jewish heritage was sparked by her interactions with working-class Jewish sweatshop workers involved in social justice struggles in the East End of London, and also by the Dreyfus affair in France. Her earliest Jewish engagement was in October 1890, when she attended a meeting of a group of Jewish socialist workers in London in order to protest against antisemitic persecution in Czarist Russia. She learned Yiddish and sometimes delivered lectures in the language.

In 1884, Eleanor joined the Social Democratic Federation (SDF), led by Henry Hyndman, and was elected to its executive. During her work in the SDF, she met Edward Aveling, with whom she would spend the rest of her life, despite his faithlessness, alleged thievery from the movement, and mental cruelty.