The ancient city of Thera (sometimes also spelled Thira) was located off the southeastern coast of the Greek island of Santorini, on the inaccessible ridge of the steep, 360-metre-high Mesa Vouno. However, the city of Thera is not to be confused with the present-day capital of Fira, which lies on the western side of the island.

The city takes its name from the mythical ruler of the island, Theras, son of the King of Thebes. In the 9th century BC, colonists from Sparta arrived on the island of Kallistos (as Santorini was also called) and decided to take advantage of the strategic location of the high mountain located in the south-east of the island. They then named the town and the island Thera in honour of their king. This settlement, though initially insignificant, soon earned a reputation as the "mother city" of Cyrene (an ancient settlement on the northern coast of Africa founded around 630 BC by the inhabitants of the island of Thera).

A long and fairly wide road ran through the centre of the city, lined mainly with public buildings. Residential buildings were clustered around the agora (the word 'agora' refers to the gathering place in the centre of the ancient city where public life took place), the eastern part of the plateau was set aside for temples and other sacred buildings, and high up in the mountains the garrison was based. All the buildings were constructed from the limestone of the mountain itself; wood was rare and so was rarely used as a building material. The town also included a harbour.

The role of the city changed in the second half of the 3rd century BC, in the so-called Hellenistic period, when the Ptolemaic war fleet for the whole Aegean Sea moved to the city port. The city was completely rebuilt to meet the needs of the officers, including the construction of a shipyard and accommodation for sailors and soldiers. The fleet was anchored here until about 140 BC.

During the period of Roman rule, which can be dated from about the middle of the 1st century BC, the city and the island were part of the Roman province of Asia, and although there were no high-ranking officials living on the island, the city managed to maintain its prosperity and considerable importance. After the 2nd century AD, a new faith came to the island - Christianity. The city of Thera remained inhabited until 726 AD, when a minor eruption of the Santorini volcano occurred and the inhabitants moved to the coast of the island in search of a more comfortable life.

The first archaeological excavations in the area took place at the turn of the 19th and 20th centuries, and were followed up in the 1960s by N. Zapheiropoulos, under the auspices of the Archaeological Society of Athens, who, together with his team, uncovered the urban necropolis at Sellada, on the outskirts of the present-day town of Kamari (a necropolis is a specific type of historical burial site, the name comes from the Greek and means "city of the dead"). Excavations were then resumed in the 1990s under the direction of the German archaeologist Wolfram Hoepfner, and the archaeological site of Thira is today one of the most important monuments of Santorini. Finds from the archaeological excavations can be seen at the Archaeological Museum of Thera or, for example, at the National Archaeological Museum in Athens.