Sălaj County (Romanian pronunciation: [səˈlaʒ]; Hungarian: Szilágy megye) (also known as Land of Silvania, silva, -ae meaning "forest") is a county (județ) of Romania, located in the north-west of the country, in the historical regions of Crișana and Transylvania. It is bordered to the north by Satu Mare and Maramureș counties, to the west and south-west by Bihor County, and to the south-east by Cluj County. Zalău is the county seat, as well as its largest city.

In Hungarian, it is known as Szilágy megye, in Slovak as Salašská župa, and in German as Kreis Zillenmarkt. The county is named after the river Sălaj, which gets its name from Hungarian Szilágy "elm creek", composed from szil, "elm" and ágy, "riverbed".

On 28 July 1978, a team of speleologists discovered in the cave of Cuciulat Paleolithic paintings about 12,000 years old, unique in Romania. Called the "Romanian Altamira", this cave features several red paintings of animals, including horses and felines. These are the first manifestations of this kind known in Southeastern Europe.

The first villages in the current territory of Sălaj County are 7,500 years old. The first ceramic pots in Sălaj area are about the same age. The first houses with several rooms were built in this county about 6,000 years ago. The only fully studied Bronze Age settlement in the Romanian territory is located in Sălaj County, in Recea.

So far, 63 bronze artifacts have been discovered dating as far back as 17th–9th centuries BC. Bronze items from this period discovered in the Sălaj County are exhibited today in museums in Germany, United States, Hungary, but also Bucharest. Six defense citadels were dated to the first Iron Age, 11th–4th centuries BC.

Between the 2nd century BC and the 1st century AD, the current territory of Sălaj was occupied by Dacians. There was a Dacian tribal union between Crasna and Barcău rivers that controlled the access roads to the north-west, to and from Transylvania, as well as the commerce, especially the salt trade. From the Dacian period come no less than 23 Dacian thesauri, made of silver coins and ornaments. The 3,000 coins and 70 silver ornaments weigh in total about 13 kg (29 lb). The largest fortified Dacian settlement in Romania was discovered in Sălaj County, dating from the 1st century AD. In total, in Sălaj County were discovered sites of 30 Dacian villages and 15 Dacian citadels defending the tribal union in the west of the current county. These citadels were located on hills and were fortified with ditches and earth walls, on which were erected wooden palisades. The center of the tribal union was on Măgura Șimleului, in a complex of settlements and fortifications.

In the western half of the county, under Roman military control, subsequently settled the Vandals, which entered into an alliance with Dacians, supported by the Romans to fight other barbarians. The Vandals arrived in the area during the 1st century AD, coming from the current territory of Denmark.

After conquering Dacia, the Romans built on the place of a Dacian settlement the capital of Dacia Porolissensis, at Porolissum (current Moigrad). The capital Porolissum had about 20,000 inhabitants, defended by the militaries in the local castra. As a work of art should be mentioned the amphitheater, a scale replica of the one in Rome, with a capacity of 6,000 seats. In 214 AD, Roman Emperor Marcus Aurelius Antoninus visited Porolissum. At Porolissum was quartered for a period Cohors III Dacorum, made of ethnic Dacians, who fought as infantry.