Acarnania (Ancient Greek: Ἀκαρνανία, romanized: Akarnanía) is a region of west-central Greece that lies along the Ionian Sea, west of Aetolia, with the Achelous River for a boundary, and north of the gulf of Calydon, which is the entrance to the Gulf of Corinth. Today it forms the western part of the regional unit of Aetolia-Acarnania. The capital and principal city in ancient times was Stratos.

Acarnania's foundation in Greek mythology was traditionally ascribed to Acarnan, son of Alcmaeon.

The name of Acarnania appears to have been unknown in the earliest times. Homer (8th century BC) only calls the country opposite Ithaca and Cephalonia, under the general name of "Epeirus" (῎ηπειρος), or the mainland, although he frequently mentions the Aetolians.

The country is said to have been originally inhabited by the Taphii (or Teleboae), the Leleges, and the Curetes. The Taphii, or Teleboae were chiefly found in the islands off the western coast of Acarnania, where they maintained themselves by piracy. The Leleges were more widely disseminated, and were also in possession at one period of Aetolia, Locris, and other parts of Greece. The Curetes are said to have come from Aetolia, and to have settled in Acarnania, after they had been expelled from the former country by Aetolus and his followers. The name of Acarnania is derived from Acarnan, the son of Alcmaeon, who is said to have settled at the mouth of the Achelous. If this tradition is of any value, it would intimate that an Argive colony settled on the coast of Acarnania at an early period.

In the 7th century BC, Greek influence in the region became prominent when Corinth settled Anactorium, Sollium and Leucas, and Kefalonia settled Astacus. The original inhabitants of the country were driven more into the interior; they never made much progress in the arts of civilised life; and even at the time of the Peloponnesian War, they were a rude and barbarous people, engaged in continual wars with their neighbours, and living by robbery and piracy. The settlements of Alyzeia, Coronta, Limnaea, Medion, Oeniadae, Palaerus, Phoitiai and Stratus are also mentioned by Thucydides, this latter city being the seat of a loose confederation of Acarnanian powers that was maintained until the late 1st century BC.

The ancient Acarnanians, however, were Greeks, and as such were allowed to contend in the great Pan-Hellenic games, although they were closely connected with their neighbors, the Agraeans and Amphilochians on the Ambracian Gulf, who were barbarian or non-Hellenic nations. Like other rude mountaineers, the Acarnanians are praised for their fidelity and courage. They formed good light-armed troops, and were excellent slingers. They lived, for the most part, dispersed in villages, retiring, when attacked, to the mountains. Strabo relates that they were united, however, in a political League (the Acarnanian League), of which Aristotle wrote an account in a work now lost (Ἀκαρνάνων Πολιτεία). Thucydides mentions a hill, named Olpae, near the Amphilochian Argos, which the Acarnanians had fortified as a place of judicial meeting for the settlement of disputes. The meetings of the League were usually held at Stratus, which was the chief town in Acarnania; but, in the time of the Romans, the meetings took place either at Thyrium, or at Leucas, the latter of which places became, at that time, the chief city in Acarnania.

At an early period, when part of Amphilochia belonged to the Acarnanians, they used to hold a public judicial congress at Olpae, a fortified hill about 3 miles (4.8 km) from Argos Amphilochicum. Of the constitution of their League, we have scarcely any particulars. We learn from an inscription found at Punta, the site of ancient Actium, that there were a council and a general assembly of the people, by which decrees were passed: Ἔδοξε τᾷ βουλᾷ καὶ τῷ κοινῷ τῶν Ἁκαρνάνων. At the head of the League, there was a strategus (Στρατηγός) or general; and the Council had a secretary (γραμματεύς), who appears to have been a person of importance, as in the Achaean and Aetolian Leagues. The chief priest (ἱεραπόλος) of the temple of Apollo at Actium seems to have been a person of high rank; and either his name or that of the strategus was employed for official dates, like that of the first Archon at Athens.

Because it is located strategically on the maritime route to Italy, Acarnania was involved in many wars. Their hatred against the Corinthian settlers, who had deprived them of all their best ports, naturally led the Acarnanians to side with the Athenians; but the immediate cause of their alliance with the latter arose from the expulsion of the Amphilochians from the town of Argos Amphilochicum by the Corinthian settlers from Ambracia, about 432 BC. The Acarnanians espoused the cause of the expelled Amphilochians, and in order to obtain the restoration of the latter, they applied for assistance to Athens. The Athenians accordingly sent an expedition under Phormio, who took Argos, expelled the Ambraciots, and restored the town to the Amphilochians and Acarnanians. An alliance was now formally concluded between the Acarnamians and Athenians. The only towns of Acarnania which did not join it were Oeniadae and Astacus.