Bryansk Oblast lies in western European Russia in the central to western parts of the East European Plain, on the divide between the Desna and Volga basins. The oblast borders with Smolensk Oblast in the north, Kaluga Oblast in the northeast, Oryol Oblast in the east, Kursk Oblast in the southeast, Chernihiv and Sumy Oblasts of Ukraine in the south, and with Gomel and Mogilev Oblasts of Belarus in the west.
The relief is a typical East European Plain landscape, with alternating rolling hills and shallow lowlands, although lowlands dominate in the western and central parts. A total of 125 rivers flow through Bryansk Oblast, with the longest one, at 1,187 kilometers (738 mi), being the Desna (a tributary of the Dnieper). Other major rivers include the Bolva, Navlya, Nerussa, Sudost (all tributaries of the Desna), Besed, and Iput. There are forty-nine major lakes, with Lake Kozhany being the largest.
Climate is temperate continental. The average temperature in January is −7 to −9 °C (19 to 16 °F). The average July temperature is +18 to +19 °C (64 to 66 °F). Average annual precipitation varies from 560 to 600 millimeters (22 to 24 in).
Natural resources include deposits of peat, sand, clay, chalk, marl, and other building materials, as well as phosphorite. About a quarter of the total area of the oblast is covered by forests, mainly coniferous, mixed, and deciduous, as well as forest-steppe.
As a result of the Chernobyl disaster on April 26, 1986, part of the territory of Bryansk Oblast has been contaminated with radionuclides (mainly Klimovsky, Klintsovsky, Krasnogorsky, Surazhsky, and Novozybkovsky Districts). In 1999, some 226,000 people lived in areas with the contamination level above 5 Curie/km2, representing approximately 16% of the oblast's population.
The Venus of Eliseevichi is a piece of Paleolithic art dated 14,000 YBP found in the region. The Eliseevichi site is also associated with the earliest recognized dog remains dating to 15,000 YBP.