Pessinus (Greek: Πεσσινούς or Πισσινούς) was an Ancient city and archbishopric in Asia Minor, a geographical area roughly covering modern Anatolia (Asian Turkey). The site of the city is now the modern Turkish village of Ballıhisar, in a tributary valley of the Sakarya River on the high Anatolian plateau at ca. 950 m above sea level, 13 km from the small town of Sivrihisar. Pessinus remains a Catholic (formerly double) titular see.

As yet, the temple area, which was excavated between 1967 and 1972, is the only well-studied area of Pessinus. It was studied thoroughly by M. Waelkens (current director of Sagalassos excavations) in the 1980s and between 2006 and 2012 by Verlinde (Ghent University), who built on the findings of the former to analyze and reconstruct the architecture of the Corinthian peripteral temple, of which only the massive foundations remain. Investigations led to several observations, such as the Tiberian date (25-35 AD) of the cult building and its identification as a temple of the imperial cult (Sebasteion). As such, it was finally established that the excavated temple could not be identified as the Temple of Cybele, as explorer Charles Texier had done when he 'discovered' the foundations of the temple in 1834. Verlinde discovered that the building was designed on the basis of a grid, and that the governing module, determining the intervals and height of the columns, was equal to the lower diameter of the columns (0.76 m). Each intercolumnar space was equal to two modules (1.52 m), which designates the temple as a 'systyle.'

Furthermore, the extraordinarily large stepped podium seems to have been influenced by Hellenistic and early Imperial pseudodipteroi. Although the temple was Tiberian, the decorative sculpture was fashioned in a conservative Augustan manner, which suggests that the building may have been design in the late Augustan period (ca. 15 AD). The temple towered over the back of a theatre, which combined a central staircase with two cavea wings for spectators. It was claimed by Verlinde that this theatrical area was ritual and used for gladiatorial fights, as the theatre contained raised seats with a protective parapet, which was typical for gladiatorial theatres in the Greek east. Given that such gladiatorial combat was as a rule intertwined with the imperial cult, Verlinde argued that the epigraphically attested cult of the emperor, was once again confirmed. He also observed that there is a consistency of such theatre-temples, which were influenced by late Republican sanctuaries in Italy (e.g. the sanctuary of Hercules Victor at Tivoli), being associated with the imperial cult. The sanctuary of Augustus at Stratonicea, which was a theatre-temple as well, may have served as a model for the sanctuary in Pessinus.

The colonnaded square in front of the stairway-theatre was thought to have been part of the imperial complex. However, this was rejected by Verlinde who dated the complex to the late 2nd century BC. The architecture of the limestone complex (covered with stucco lustro) emanates the style of Hellenistic palaestrae such as the Gymnasion of Eudemos at Miletus (late 3rd century BC). Being quite similar to the latter complex, the Pessinuntian square was reconstructed by Verlinde as a 'quadriporticus' with a Rhodian peristyle, that is with a high (Ionic) colonnade to the north, and three lower wings with Doric columns. The quadriporticus was an annex of the Hellenistic citadel on the promontory to the east, which preceded the early imperial temple.

The combination of a Hellenistic palace and a gymnasium (school) was a typical phenomenon of the Greek world during the Hellenistic age. Carbon dating and ceramological analysis indicates that the palaestra (sports gym) was destroyed by a fire during the late Hellenistic age, suggesting that the colonnaded square as a functional entity was short-lived. After the quadriporticus was destroyed, it was not rebuilt during the early Roman period, as the area may have been used as an unpaved arena for the gladiatorial fights of the temple. In the 3rd century AD, the area was monumentalized with a new ellipse-shaped theatre and a vast marble square with a monumental funerary crypt (a funerary Heroon). This coincided with the further monumentalization of the cardo maximus, which received monumental city gates in the form of arches at its southern and northern extremity.

The mythological King Midas (738-696 BC?) is said to have ruled a greater Phrygian realm from Pessinus, but archaeological research since 1967 showed that the city developed around 400 BC at the earliest, which contradicts any historical claim of early Phrygian roots.

According to ancient tradition, Pessinus was the principal cult centre of the goddess Cybele, the Phrygian Meter ("Mother"). Tradition situates the cult of Cybele in the early Phrygian period (8th century BC) and associates the erection of her first "costly" temple and even the founding of the city with king Midas (738-696 BC?). However, the Phrygian past of Pessinus is still obscure, both historically as archaeologically. For example, the geographer Strabo (12.5.3) writes that the priests were potentates in "ancient times", but it is unclear whether Pessinus was already a temple state ruled by dynastai ("lords") in the Phrygian period.

By the 3rd century BC at the latest, Pessinus had become a temple state ruled by a clerical oligarchy consisting of Galloi, eunuch priests of the Mother Goddess. After the arrival of Celtic tribes in Asia Minor in 278/277 BC, and their defeat at the hand of Antiochus I during the so-called 'Battle of the Elephants' (likely 268 BC), the Celts settled in the north-central region of Anatolia which became known as Galatia. The tribe of the Tolistobogii occupied the Phrygian territory between Gordium and Pessinus. It is doubtful that the temple state actually stood under Galatian control at this early stage. According to Cicero (Har. Resp. 8.28) the Seleucid kings held deep devotion for the shrine.