partido (county) of eastern Gran (Greater) Buenos Aires, eastern Argentina. It lies immediately southeast of the city of Buenos Aires, in Buenos Aires provincia (province), on the Río de la Plata estuary. In the late 19th and early 20th centuries the area was an industrial slum, slaughterhouse, and port district, separated from Buenos Aires by the Riachuelo River. Early settlers were Spanish, Italian, and Polish immigrants. The county was formally established in 1852, when the governor of Buenos Aires province, Vicente López, expropriated land from the existing county of Quilmes. The county was renamed in 1914 in honour of Nicholás Avellaneda, former president of Argentina (1874–80). The county experienced rapid commercial development in the 20th century, based on the processing and marketing of hides, wool, and meat.

During the mid-1940s Avellaneda became a centre of Peronist activity. In 1945 the descamisados (“shirtless ones”), rural migrants to the area, demonstrated in Perón's behalf, demanding his return from exile.

Port facilities at Avellaneda are utilized mostly for coastal and river trade and continue to concentrate mostly on the handling of wool, hides, and other animal produce. Pop. (2001) 328,980.