Ainis (Ancient Greek: Αἰνίς, IPA: [ajˈniːs], Modern Greek Αινίδα, IPA: [eˈniða]) or Aeniania (Αἰνιανία), was a region of ancient Greece located near Lamia in modern Central Greece, roughly corresponding to the upper valley of the Spercheios river.

The region takes its name from the tribe of the Ainianians, who dwelt in the area. The name Ainis first occurs in Roman times; the only known earlier name of the region was "land of the Aenianians", Ainianōn khōra (Theopompus).

Ainis is located in the upper Spercheios valley, bordering with Dolopia in the west, Oitaia in the south, Malis in the east and Achaia Phthiotis in the north. The exact borders with Oitaia and Malis have never been established. The river Spercheios flows through the region on its way down to the Maliac Gulf, and is joined in Ainis by its chief tributary the Inachos. The area is limited to the north by the Othrys mountains, and to the west by a spur of the Pindus mountains, with the peak of Tymphrestus visible from most of the region. To the south lie the peaks of Mount Goulinas and Mount Oiti, separated by the river Inachos.

Most of Ainis consists today of a fertile river plain; whether this was the case during Antiquity does however remain uncertain. As with Greece in general, there is some seismic activity with hot springs close to the village of Platystomo. After the introduction of modern heating, the previously bald foothills of the surrounding mountains are now covered with dense thickets of ivy and prickly pear.

Plutarch writes (Quest. Graec. 12) that the Ainianians were once expelled from Thessaly by the Lapiths to wander the Greek peninsula until they finally settled in the upper Valley of Spercheios. According to Plutarch, when the Ainianians finally settled in what would become Ainis, the land was already occupied by the Inachians and the Achaeans. Phemios, king of the Ainianians, however, killed Hyparochos king of the Inachians with a stone while the latter had his head turned, thus winning the region for his people.

The Ainianians struck coins in Hypata with the head of Zeus on the obverse and the legendary king Phemios on the reverse.

Very little is known of the settlements in ancient Ainis apart from the city of Hypata. Several poleis (Kapheleis, Korophaioi, Phyrrhagioi and Talana) are mentioned in inscriptions at Delphi, but apart from Hypate, none has yet been convincingly identified. Moreover, other settlements that were not poleis have been identified, including Sosthenis and Spercheiai. A smaller settlement, that of Makra Kome, is also mentioned in a passage by Livy as being ravaged by the Aetolians during the Second Macedonian War. There are, however, several known sites in the area, mostly of the Hellenistic period, some of them of urban character.

The "capital" of Ainis, Hypata (Ὑπάτα), was located at the modern town of Ypati on the northern slope of Mount Oeta; the name is probably derived from a corruption of hypo Oita (ὑπὸ Οἴτα, meaning "near the Mount Oeta"). The ancient city was probably divided in a lower fortified city located approximately at the modern location of the town, and an acropolis which is still visible further up the mountain. Apart from some fortifications and inscriptions, very little of the ancient town is visible today; travelers in the early 20th century noted ancient blocks and slabs built into the modern houses, but most of these were probably destroyed when the German Wehrmacht demolished most of the town during the Second World War. The commanding tower on the acropolis is of a later date, built in the Middle Ages. The political area of Hypata probably extended far north on the river plain, which is also mentioned in several inscriptions. A road over Mount Oeta led southward from Hypata towards Kallion in Aetolia.