Thermos (/ˈθɜːrmɒs/; also known as Thermon /ˈθɜːrmən/, Thermum; Ancient Greek: Θέρμος or Thermika) was an ancient Greek sanctuary, which served as the regular meeting place of the Aetolian League. Its focal point was the temple of Apollo Thermios of about 630 to 610 BC, "one of the earliest developed Doric temples known and a monument of primary importance for our knowledge of the history of Greek architecture". The most famous survivals are the Archaic terracotta metopes decorated with painted scenes from mythology, which are among the earliest examples of this art form in Greece. What is left of these, and other finds from the site, are now in the museum at Thermos, with a selection of the best pieces in National Archaeological Museum, Athens.

Thermos was not a city in the sense of a built-up urban centre like Athens, Argos or Corinth and until a late date the Aetolian League was a loose association with a tribal basis rather than a group of city-states.

The ancient name is preserved in the nearby contemporary Greek village of Thermo, Greece.

Because of the gradually diminishing significance of the site, this very early temple of Archaic Greece was never fully rebuilt and expanded, as happened to most of its contemporaries. It has one of the best preserved pterons, and "is the first of the great tiled buildings" to survive with readable remains. Originally the walls were mud brick, the columns wood, and the entablature wood decorated with painted terracotta. The roof was tiled in terracotta, a recent innovation for the Greeks; the extra weight compared to thatch or wooden shingles was perhaps a factor driving the change to building with stone.

Much later, in the Hellenistic period, the wooden columns were replaced with stone, but the entablature seems to have been left. There were fifteen columns on each side, and five at each end (counting the corner ones twice), also a row of ten columns down the cella. The survivals in terracotta are metopes, nearly three feet square, and mask antefixes above them. The overall dimensions are c. 40 x 125 ft (12.13 x 38.23 m). It had a very early and rather deep example of an opisthodomos (rear room) with two columns down the centre. The tiled roof had a gable at the front, but sloped down at the rear.

There are "an elaborate series of terracotta revetments found scattered about the temple site. These include roof tiles, simas, at least two series of antefixes decorated in relief with human busts, a sphinx acroterion and 10 fragmentary metope plaques with painted representations." The metopes include a large gorgon head and various mythological subjects, inside vertical borders of rosettes. One of the best preserved shows Procne/Aëdon and Philomela/Chelidon preparing Itys/Itylus for the dinner table. Their similarity in style to Corinthian painted pottery of around the 630s BC is the main basis for the dating of the temple, though the clay is local, and the potters may also have been. Some are "inscribed in a mixed alphabet which may well be Aitolian".

There are fragments of two sets of antefixes, the second set with heads of men and silenes suggesting that the roof was partly renewed or remodelled in the mid-6th century.

Together with the stone metopes of "Temple C" at Selinus in Sicily, of about 550 BC, the metopes of Thermos have been brought into the long-running scholarly discussions over the extent to which the Doric order in stone recreates or imitates earlier buildings in wood. The two sets "are commonly held to be the oldest metopes known so far". It is argued that both sets are too large ("way too high"), at about 90 cm, for the function traditionally assigned to them in the wood-to-stone model, but instead demonstrate that "the Doric frieze was a decorative rather a structural feature" from the start.