Irkutsk Oblast is a federal subject of Russia, located in southeastern Siberia in the basins of the Angara, Lena, and Nizhnyaya Tunguska Rivers. The administrative center is the city of Irkutsk. It borders the Republic of Buryatia and the Tuva Republic in the south and southwest, which separate it from Khövsgöl Province in Mongolia; Krasnoyarsk Krai in the west; the Sakha Republic in the northeast; and Zabaykalsky Krai in the east. It had a population of 2,370,102 at the 2021 Census.

The Angara is a major river in Siberia, which traces a course through Russia's Irkutsk Oblast and Krasnoyarsk Krai. It drains out of Lake Baikal and is the headwater tributary of the Yenisey. It is 1,849 kilometres (1,149 mi) long, and has a drainage basin of 1,039,000 square kilometres (401,000 sq mi). It was formerly known as the Lower or Nizhnyaya Angara. Below its junction with the Ilim, it was formerly known as the Upper Tunguska and, with the names reversed, as the Lower Tunguska.