Byllis (Albanian: Bylis; Greek: Βύλλις; Latin: Byllis) or Bullis or Boullis (Βουλλίς) was an ancient city and the chief settlement of the Illyrian tribe of the Bylliones, traditionally located in southern Illyria. In Hellenistic times the city was either part of Illyria or Epirus. In Roman times it was included within Epirus Nova, in the province of Macedonia. The remains of Byllis are situated north-east of Vlorë, 25 kilometers from the sea in Hekal, Fier County, Albania. Byllis was designated as an archaeological park on 7 April 2003 by the government of Albania.

The massive walls of Byllis were built around 350 BC when the Illyrians went through a dynamic development founding their own cities. The urban settlement was built on the territory of the community of the Bylliones on an already existent Illyrian hilltop proto-urban area dating back to the previous century. Later Byllis acquired the trappings of a Hellenistic town, and because the southernmost Illyrian tribes, including the Bylliones, were inclined to become bilingual, it was also a Greek-speaking city. Byllis received sacred ancient Greek envoys, known as theoroi, during the early 2nd century BC, which only cities that were considered Greek were eligible to receive. The time duration that passed before Illyrian cities were documented on a list of theorodokoi clarifies that acculturation did take place in southern Illyria, however it indicates that the process was gradual.

During the Roman-Illyrian war in 169/168 BC the Bylliones took part on the Roman side against the Illyrian king Gentius. However, the subsequent alliance of Byllis with Molossians and Macedonians led to its sacking and destruction by the Romans. After a long decline, in 30 BC the city became a Roman colony. In Roman times Byllis rose again, also becoming a bishopric in late antiquity.

The toponym is attested as Βύλλις, η on epigraphic material from the 3rd-2nd centuries BC, and as Βουλλίς Boullis by Ptolemy in his Geography. The city ethnic is attested as Βυλλίων Byllion on the inscription of an oracular lead tablet from Dodona dating back to the 4th century BC and on coins of the Hellenistic era dating back to the 3rd-2nd centuries BC, or as Βουλινός Boulinos by Pseudo-Scymnus. The city is attested as a polis exclusively in the work of Stephanus of Byzantium in the 6th century AD, who mentions it as πόλις 'Ιλλυρίδος polis Illyridos, and its ethnicon as Βυλλιδεύς Bullideus. Stephanus of Byzantium mentions a town called Byllis as a seaside city in Illyria and its foundation legend, according to which the city was supposedly built by Myrmidons under Neoptolemus, returning from the Trojan War towards its homeland. This legendary tradition of the city is reflected also by numismatics of the Hellenistic era, however it is contradicted by archaeological research.

It has been suggested that the city had its own ethnic, Βυλλιδεύς, with coinage which bore the legend ΒΥΛΛΙΣ minted separately from the coinage of the tribe of the Bylliones which bore the legend ΒΥΛΛΙΟΝΩΝ. However both those names appear on coins that were issued by the koinon of the Bylliones. A recent analysis of the epigraphy and numismatics of the area seems to indicate an alternating use of the names: the ethnos with its territory on the one hand, and the "tribal" polis that was the capital of the ethnos on the other hand, being internally organized with a political authority represented by the decision-making bodies of the koinon of the Bylliones, while representing the urban center of reference for the community.

Byllis was founded on the territory of the Illyrian community of the Bylliones around 350 BC, on an already existent proto-urban area dating back to the previous century. Initially it was one of the several fortified Illyrian hilltops that controlled the high hills of the region of Mallakastër. The area was on the edge of the sphere of influence by Apollonia and Epidamnos, as well as of Epirus. The foundation of the Illyrian city in the 4th century on the northern edge of the Vjosa valley is probably connected with the presence of bitumen mines near Selenica, which were located on the opposite edge of the valley.

Mid-4th century BC fortification walls were built with isodomic ashlar layers. The walls were 2.25 km long and 3.5 m wide, enclosing an area of 30 hectares around a steep hill. In the sanctuary of Dodona a 4th-century BC inscription on a lead foil provides the earliest known attestation of the Bylliones, asking to which deity they should sacrifice in order to ensure the safety of their possessions.

The city experienced a notable development in the Hellenistic period, representing the chief settlement of the Illyrian koinon of the Bylliones, of which several hilltop centers are found in the lower valley of the Vjosa river. The archaeological remains confirm that in Hellenistic times Byllis was a cultural and political centre of the region. The creation of Hellenistic cities such as Byllis in the territory of southern Albania has been attributed to the successors of Alexander the Great and Pyrrhus of Epirus.