The area of the county does not form a geographical unit; it shares several features with the neighbouring areas: from northeast the sand hills of the Nyírség spread over the county borders. The western part is the Hortobágy National Park ("Puszta"), a large flat area of the country.
The county of Hajdú-Bihar occupies the eastern part of Hungary. Most of its territory is completely flat and is part of the Pannonian Plain region (called the Grand Plain in the country). The highest point hardly rises over 170.5 metres in the north. It seems that the county slopes to the south because the lowest point is situated in this part of Hajdú-Bihar and hits the 85 metres height.
The wind and rivers formed and shaped the land for thousands of years. First the area of the present-day Hungary was occupied by an inland sea. Then, after some underground movement, the huge peaks of the Carpathians rose from this sea. The wild and rapid rivers of the mountains slowly made that inland sea disappear. Later, the great Hungarian plain was formed by the alluvial deposits of the rivers, the wind began to work and from the great rocks became smaller and smaller sand-grains, the so-called loess. This covers thickly the Hajdú-Bihar plain as well and makes a fertile soil.
There are two great rivers in this area, the Tisza and the Körös. The Hortobágy area was formed by the first one. It used to be the flood area of the Tisza river and after the river was controlled, the Hortobágy became dry, resulting in special fauna.
Highways no. 4, 33, 35, 42 and 47 lead to the county and the county can also be reached Hajdú-Bihar via the M3 motorway (this motorway is now a spur to Debrecen). The total length of the public roads in the county is 1,511 km. 122 road bridges span the rivers and canals. The roads cross the national borders to Romania at the cities of Ártánd and Nyírábrány.
Compared to the average, Hajdú-Bihar has a small number of inhabited places: 19 towns and 62 villages. The four largest cities – both by area and by population – are Debrecen, Hajdúböszörmény, Hajdúnánás and Hajdúszoboszló. For a long time these were the only ones with city/town status, the rest were only villages, but after 1979 – when Berettyóújfalu became a town – several villages were promoted.