Julius Popper (December 15, 1857 – June 5, 1893), known in Spanish as Julio Popper (Spanish pronunciation: [ˈxuljo poˈpeɾ]), was a Romanian-born Argentine engineer, adventurer, and explorer. He was known as a modern "conquistador" of Tierra del Fuego in southern South America, and was both a controversial and influential figure. Popper was one of the main perpetrators of the genocide against the native Selk'nam people in the islands, and the circumstances surrounding his own death remain a mystery.
 Popper was born in 1857 to a Jewish family in Bucharest, Principality of Wallachia, son of professor Neftali Popper, a successful antiques merchant, and his wife Peppi. He studied in Paris, gaining credentials as an engineer.
 After working in Europe for several years, he took a job working on the infrastructure for the telegraph in Chile. He arrived in Argentina in 1885, where he was attracted by the possibility of gold mining in Tierra del Fuego. In 1886 he received a permit from the Argentine Government to form an exploration company to mine for gold near San Sebastián. On September 7, he led an 18-man expedition that included a chief engineer, a mineralogist, a journalist and a photographer. They found gold dust on the beach of El Páramo, in San Sebastián Bay. The expedition was rigorously and strictly enforced according to military standards with heavily armed men, with Popper in direct command of everything. 
 During the expedition, Popper and his men were allegedly attacked by eighty Selk'nam (Ona) armed with bows. The adventurers responded by firing their Winchester rifles, killing all but two of the Selk'nam. After the fight, Popper "posed his men in the attitude of troops repelling a charge, took a position himself astride one of the dead Indians, and then had the outfit photographed for subsequent use."  
 Popper succeeded in unearthing large amounts of gold and his Compania de Lavaderos de Oro del Sud realized enormous capital gains on the Argentine stock exchange. A mint built to manage the gold was adapted as a museum in 1973, Museo del Fin del Mundo  [es] ("Museum at the End of the Earth"), officially the Museo Territorial (Territorial Museum) of Tierra del Fuego since 1979.
 In Patagonia, Popper maintained dominance with his private army. He issued his own coins and stamps to symbolize his power. When the Argentine peso lost its value in the market crash of 1890, his gold coins were regarded as currency. Around this time, he may have produced plans for the modern outline of the city of Havana, Cuba.
 Popper vigorously fought against his enemies; he punished gold diggers and thieves according to arbitrary law. The most controversial aspect of his life was his participation in the Selk'nam genocide against the native communities on Tierra del Fuego. Sheep farmers and gold miners ruthlessly killed them; the former because the Selk'nam would hunt sheep in their former territories and the latter because of conflicts over mining areas. Together with other bounty hunters, who were paid to kill the Selk'nam, Popper too sent his armed forces to manhunt them.
 Popper also prepared an expedition to enforce the Argentine claim for parts of Antarctica.
 After Popper's sudden death in Buenos Aires at the age of 35, his empire collapsed. The cause of his death has not been established. Contemporary American journalist John R. Spears says that he was poisoned by "men whom he had offended in the south." Popper's death was seen as suspicious due to his relatively young age and good health.
 In July 2022 The Wilhelm Filderman Center for the Study of the History of the Jews of Romania mounted in the Romanian capital, Bucharest, an exhibition of Popper's 1886 expedition into the interior of Tierra del Fuego. It consisted of a selection of the hundreds of photographs of the expedition that Popper himself sent to his family in Bucharest at the time and which collection had previously been conserved in the Romanian National Archives.