The Molossians (Greek: Μολοσσοί or Μολοττοί, romanized: Molossoi or Molottoi) were a group of ancient Greek tribes which inhabited the region of Epirus in classical antiquity. Together with the Chaonians and the Thesprotians, they formed the main tribal groupings of the northwestern Greek group. On their northern frontier, they neighbored the Chaonians and on their southern frontier neighbored the kingdom of the Thesprotians. They formed their own state around 370 BC and were part of the League of Epirus. The most famous Molossian ruler was Pyrrhus of Epirus, considered one of the greatest generals of antiquity. The Molossians sided with the Macedonians against Rome in the Third Macedonian War (171–168 BC) and were defeated. Following the war, the region witnessed devastation while a considerable number of Molossians and other Epirotes were enslaved and transported to the Roman Republic, in the Italian Peninsula.

According to Strabo, the Molossians, along with the Chaonians and Thesprotians, were the most famous among the fourteen tribes of Epirus, who once ruled over the whole region. The Chaonians ruled Epirus at an earlier time, and afterwards the Thesprotians and Molossians controlled the region. The Thesprotians, the Chaonians, and the Molossians were the three principal clusters of Greek tribes that had emerged from Epirus and were the most powerful among all other tribes.

The Molossians were also renowned for their vicious hounds, which were used by shepherds to guard their flocks. This is where the canine breed Molossoid, native to Greece, received its name. Virgil tells us that in ancient Greece the heavier Molossian dogs were often used by the Greeks and Romans for hunting (canis venaticus) and to watch over the house and livestock (canis pastoralis). "Never, with them on guard," says Virgil, "need you fear for your stalls a midnight thief, or onslaught of wolves, or Iberian brigands at your back."

Strabo records that the Thesprotians, Molossians and Macedonians referred to old men as pelioi (πελιοί) and old women as peliai (πελιαί) (<PIE *pel-, "grey"). Cf. Ancient Greek πέλεια peleia, "pigeon", so-called because of its dusky grey color. Ancient Greek πελός pelos meant "grey". Their senators were called Peligones (Πελιγόνες), similar to Macedonian Peliganes (Πελιγᾶνες).

The Molossian ruling dynasty claimed to be descended from mythological Molossus, one of the three sons of Neoptolemus, son of Achilles and Deidamia. Following the sack of Troy, Neoptolemus and his armies settled in Epirus where they joined with the local population. Molossus inherited the kingdom of Epirus after the death of Helenus, son of Priam and Hecuba of Troy, who had married his erstwhile sister-in-law Andromache after Neoptolemus's death. According to some historians, their first king was Phaethon, one of those who came into Epirus with Pelasgus. According to Plutarch, Deucalion and Pyrrha, having set up the worship of Zeus at Dodona, settled there among the Molossians. At the time, among writers of the classical era these stories were not doubted . According to Johannes Engels (2010) in the Oxford Companion to Macedonia, genealogical links through the Trojan cycle and other myths strongly connected Epirus with the rest of Greece, precluding serious debate about the Greekness of the Epirotes, including the Molossians.

The most famed member of the Molossian dynasty was Pyrrhus, who became famous for his several Pyrrhic victories in battle over the Romans. According to Plutarch, Pyrrhus was the son of Aeacides of Epirus and a Greek woman from Thessaly named Phthia, the daughter of a war hero in the Lamian War. Pyrrhus was a second cousin of Alexander the Great. Moreover, Olympias, the mother of Alexander the Great, was a member of this celebrated sovereign house.

During the Late Bronze Age the Molossians were probably located over much of the central and western ranges of the Pindos. They were among the known Greek tribes of the Mycenaean period (1600–1100 BC) after the Proto-Greek split. The area of Pogoni has been regarded as the heartland of the Molossian tribes due to the large number of tumuli burials found in this region dating from that time. They initially lived in small unwalled settlements, kata komas, mainly scattered in the river valleys and lakeside areas of central Epirus. Among those settlements the most excavated were located in Liatovouni at the confluence of the Aoös and Voidomatis rivers at the valley of Konitsa established in the 13th–12th century B.C and Vitsa (from 9th century B.C). At the earliest burial at Liatovouni Mycenaean weapons were unearthed. This Molossian cemetery consist of a total of 103 burials and was in use until late 5th to early 4th century B.C. A large Molossian cemetery, was also found at Koutsokrano, Pogoni.

Molossians were also among the Greek colonists that reached the Ionian shore of Asia Minor during the period of its colonization (1020–900 BC). The Molossian expansion in Epirus possibly began in the early 6th century BC. As such they became a leading power in the region already from the time of historian Hecataeus (c. 550–476 BC). Their expansion was primarily directed towards the Thesprotians. However, the nearby Chaonians also lost some pastures but they kept control of an area stretching from Grammos (ancient Boion) to the southwest of Ohrid-Prespa.