Cluj County (Romanian pronunciation: [kluʒ] ^ⓘ) is a county (județ) of Romania, in Transylvania. Its seat is Cluj-Napoca.

In Hungarian it is known as Kolozs megye. Under the Kingdom of Hungary, a county with an identical name (Kolozs County, Romanian: Comitatul Cluj) existed since the 11th century.

Cluj County lies in the northwestern half of the country, between parallels 47°28' in north and 46°24' in south, meridians 23°39' in west and 24°13' in east, respectively. It covers an area of 6,674 km² (2,577 sq mi) unfolded in the contact zone of three representative natural units: the Apuseni Mountains, the Someș Plateau, and the Transylvanian Plain. Cluj County is the 12th largest in the country and occupies 2.8% of Romania's area. It is bordered to the northeast with Maramureș and Bistrița-Năsăud counties, to the east with Mureș County, to the south with Alba County, and to the west with Bihor and Sălaj counties.

The relief is rugged, constituted mostly of hilly units belonging to Someș Plateau and, to a lesser extent, of mountain portions that represent the northeastern part of Apuseni Mountains. Fields are missing on the territory of Cluj County, being replaced, as step of relief, by well-developed terraces and floodplains in the lower sectors of the Someșul Mic and Arieș rivers. The hilly area, extended in the central northern and southern part of the county, include several subunits of the Someș Plateau (Cluj, Feleacu, Dej hills), among which are individualized numerous depressions (Apahida, Bonțida, Gilău, Dej, Turda, Câmpia Turzii), at the edge of which there are some contact depressions (Huedin and Iara), at the limit of the mountain. The mountain sector, located in the southwest, belongs to the Apuseni Mountains, a mountain group of the Western Carpathians, represented by sectors of Vlădeasa Massif (peak Vlădeasa – 1,836 m), Gilău Mountains, and Muntele Mare, as well as narrow portions of Trascău, Plopiș, Meseș, and Bihor mountains. In the eastern part of Cluj County, east of the river alignment Valea Florilor–Maraloiu–Someșul Mic, is located a part of the Transylvanian Plain, a special unit of relief, with mountainous character, partially overlapped by some gas-condensate domes.

The hydrographic network is represented by rivers (Someșul Mic with SW–NE direction, whose catchment area is fully developed on the territory of Cluj County, Crișul Repede, and lower Arieș), natural lakes and ponds (Cătina Popii I and II, Geaca, Țaga, etc.) and lakes of hydropower interest (Beliș-Fântânele, Tarnița and Gilău). In the northeastern part of Cluj County, Someșul Mic joins Someșul Mare, upstream of Dej, forming the Someș, which flows into the Tisza to the west.

The deep waters are less represented and are characterized by high mineralization. Relatively rich mineral springs, with sulphates, calcium and sodium chloride, can be found in Dezmir, Cojocna, Gădălin, Sic, Gherla, Leghia, Someșeni, Turda, etc.

Through its location, Cluj County benefits from a moderate continental climate. In the mountain sector (Vlădeasa Massif and Muntele Mare), the mean annual air temperature is 2 °C, and in the rest of the territory is 6 °C. Annual thermal amplitudes have values between 17 and 19 °C in the mountains and grow at 23–25 °C in hilly areas, plateaus and plains.

Rainfall is distributed unevenly, with minimal amounts in Turda–Câmpia Turzii (about 550 mm) and maximum in the Vlădeasa Massif (about 1,400 mm). On average, annual values of rainfall reach 600–650 mm in the Transylvania Plain, 650–700 mm in the Someș Plateau and over 900–1000 mm in mountainous areas.