In antiquity, Paeonia or Paionia (Ancient Greek: Παιονία, romanized: Paionía) was the land and kingdom of the Paeonians (or Paionians; Ancient Greek: Παίονες, romanized: Paíones).

The exact original boundaries of Paeonia, like the early history of its inhabitants, are obscure, but it is known that it roughly corresponds to most of present-day North Macedonia and north-central parts of Greek Macedonia before the expansion of Macedon (i.e. probably the Greek municipalities of Paionia (excluding the village of Evropos), Almopia, Sintiki, Irakleia, and Serres), and a small part of south-western Bulgaria. Ancient authors placed it south of Dardania (an area corresponding to modern-day Kosovo and northern North Macedonia), west of the Thracian mountains, and east of the southernmost Illyrians. It was separated from Dardania by the mountains through which the Vardar river passes from the field of Scupi (modern Skopje) to the valley of Bylazora (near modern Sveti Nikole).

In the Iliad, the Paeonians are portrayed as allies of the Trojans. During the Persian invasion of Greece, the conquered Paeonians from as far as the Lake Prasias, including the Paeoplae and Siropaiones, were deported from Paeonia to Asia.

In 355–354 BC, Philip II of Macedon took advantage of the death of King Agis of Paeonia, and campaigned against its northern neighbor in order to conquer it. Down to 227 BC, at least seven Paeonian kings reigned, and during that period, Paeonia remained a distinct entity, either subject to Macedonia or independent. In 227 BC, Antigonus Doson conquered the southern part of Paeonia (becoming 'Macedonian Paeonia') and founded the city of Antigoneia (near modern Negotino), and ten years after Philip V of Macedon conquered the rest by capturing Bylazora; after this Perseis and Astraion were founded. The citizens of Paeonian cities were not Macedonians but Paeonians. Only after the Roman conquest in 168 BC, Paeonia was integrated into Macedonia. Pelagonia was used as a name for the westernmost part of Paeonia, while the north-westernmost part of Pelagonia was referred to as Derriopos.

The Paeonian tribes were:

There is relatively little mention of the Paeonians in the works of the ancient Greeks. Some modern scholars consider the Paeonians to have been of either Illyrian, Phrygian, Thracian, or of mixed origins. Some Paeonian toponyms and personal names of individual Paeonians are definitely Hellenic (Lycceius, Ariston, Audoleon). Some scholars have proposed a Greek (Hellenic) origin. According to Radoslav Katičić, the possibility that the Paeonians took part in the "great Greek migration" and remained behind on the route cannot be wholly ruled out. Linguistically, the very small number of surviving words in the Paeonian language have been variously connected to its neighboring languages – Illyrian and Thracian (and every possible Thraco-Illyrian mix in between), Phrygian, as well as to Greek but with a great deal of Illyrian and Thracian influence as a result of their proximity. Several eastern Paeonian tribes, including the Agrianes, clearly fell within the Thracian sphere of influence. Yet, according to the national legend, they were Teucrian colonists from Troy. Homer speaks of Paeonians from the Axios fighting on the side of the Trojans, but the Iliad does not mention whether the Paeonians were kin to the Trojans, and instead connects them to the Phrygians. Herodotus and Thucydides distinguish the Paeonians from the Thracians. Appian wrote of a genealogy in which Paion, the eponym of the Paeonians, is the son of Autarieus, the eponym of the Autariatae, and father of Skordiskos and Triballos, the eponyms of two central Balkanic tribes, one Celtic and the other Thracian. This might connect the Paeonians with the Illyrian complex, although as Katičić suggests, Appian might not refer to the Paeonians but might refer instead to the Pannonians, since Appian uses the Paeonian name to denote that ethnic group as well. Pausanias tells us of another genealogy, which connects the Paeonians with the Peloponnesian Epeians; Paion is said to be the son of Endymion and brother of Epeius and Aitolus. This version, indeed, establishes a Greek affiliation for the Paeonians. Homer calls the Paeonian leader Pyraechmes (parentage unknown); later on in the Iliad (Book 21), Homer mentions a second leader, Asteropaeus, son of Pelagon.

Before the reign of Darius Hystaspes, they had made their way as far east as Perinthus in Thrace on the Propontis. At one time all Mygdonia, together with Crestonia, was subject to them. When Xerxes crossed Chalcidice on his way to Therma (later renamed Thessalonica), he is said to have marched through Paeonian territory. They occupied the entire valley of the Axios (Vardar) as far inland as Stobi, the valleys to the east of it as far as the Strymon and the country round Astibus and the river of the same name, with the water of which they anointed their kings. Emathia, roughly the district between the Haliacmon and Axios, was once called Paeonia; and Pieria and Pelagonia were inhabited by Paeonians. As a consequence of the growth of Macedonian power, and under pressure from their Thracian neighbors, their territory was considerably diminished, and in historical times was limited to the north of Macedonia from Illyria to the Strymon.

In Greek mythology, the Paeonians were said to have derived their name from Paeon the son of Endymion. Endymion of Elis, the lover of the goddess of the Moon (Selene), had three sons: Paeon, Epeios, and Aetolus; the eponymous ancestors of the Paeonians, Epeians, and Aetolians, respectively. Endymion, in order to give his kingdom to one of them, made them run a race in Olympia, where Epeios won and took the kingdom. Paeon left in disappointment to settle in the Upper Valley of Axios which was since called Paeonia.