Porolissum was an ancient Roman city in Dacia. Established as a military fort in 106 during Trajan's Dacian Wars, the city quickly grew through trade with the natives and became the capital of the province Dacia Porolissensis in 124. It is one of the largest and best-preserved archaeological sites in modern-day Romania from the Roman Era. It is 8 km away from the modern city of Zalău, in Moigrad-Porolissum village, Mirsid Commune, Sălaj County.

Two large Dacian hillforts, part of a group of fortifications in the area, were at Poguior hill and Măgura hill, near where the castra were later built.

In 106, at the beginning of his second war against the Dacians, Emperor Trajan established a military stronghold at the site to defend the main passageway through the Carpathian Mountains. It was the centre of defence of north-west Dacia and the Limes Porolissensis. The soldiers stationed here were granted Roman citizenship by Trajan.

The garrisons of Porolissum at different times were from various parts of the empire: Cohors I Brittonum milliaria Ulpia Torquata pia fidelis civium Romanorum (nominally 1000 strong), Cohors V Lingorum, Numerus Palmyrenorum Porolissensium Sagittariorum civium Romanorum (which later became Ala Palmirenorum Porolissensium) and Cohors I Palmirenorum Porolisensium.

The vicus (civilian settlement) developed on the south and west terraces of the fort. When Hadrian created the new province Dacia Porolissensis (named for the now sizable city) in 124, Porolissum became the administrative centre of the province. Under emperor Septimius Severus, the city was granted municipium status, allowing its leaders and merchants to act independently. It became an important centre of trade with the barbarians and was probably a customs station, as the main road that started at the Danube and linked the most important centres of Dacia passed to barbaricum with evidence that heavy wagons were carrying goods in and outside the province through here.

The garrison of Porolissum seems to have lived in peaceful coexistence with their immediate neighbours. At a distance of 5–15 km from the watchtowers connected to the city several villages were apparently founded on the surrounding hills. Archaeological culture findings indicate a mixture of free populations, Dacian-Germanic, possibly Buri. There are also some inscriptions mentioning city officials with Romano-Dacian names, indicating close cooperation on a political level.

Limited archaeological work at Porolissum began in the 19th century, but it was not until 1977 when Romanian archaeologists began larger-scale, systematic excavations. The excavations by a number of teams are ongoing and have uncovered remnants of both the military installations and the civilian city, including public baths, a customs house, a temple to Liber Pater, an amphitheatre, insula consisting of four buildings and a number of houses. The main gate (Porta Praetoria) of the stone fortress has been rebuilt. A joint American-Romanian team, the Porolissum Forum Project, excavated an area of the civilian settlement from 2004–2011; despite the name of the project, the team confirmed that while this area served a public function, it was not necessarily a forum.

From 2006 until 2011, another project, "Necropolis Porolissensis", was running focused on the cemetery of the municipium Porolissum, on the spot known as "Ursoies". From 2008 to 2011 a Romanian-German-Hungarian team was excavating an underground-building in the centre of the castle, probably a water cistern.