Juliopolis or Ioulioupolis (Greek: Ἰουλιούπολις), occasionally also Heliopolis (Ἡλιούπολις), was an ancient and medieval city and episcopal see in Anatolia (modern Turkey). In later Byzantine times, it also bore the name Basilaion (Βασιλαίον). Various authors assign it to the regions of Galatia, Bithynia, and Paphlagonia.
Archaeological evidence at the site points to settlement since prehistoric times. The town was originally known as Gordiou Kome (Γορδίου Κώμη, "village of Gordion"). Cleon of Gordiucome, a native of the town, raised its status to a city and renamed it as Juliopolis in honour of the Emperor Augustus. Augustus had rewarded him with the sovereignty over Comana (Cappadocia) for his services in war against Mark Antony, whom Cleon had earlier served and from whom he had received other lands.
In late antiquity, the town gained in prominence due to its location on the so-called "Pilgrim Road" that connected Constantinople with Ancyra. It is attested as a bishopric since the Council of Ancyra in 314. Emperor Justinian I (r. 527–565) undertook repairs to strengthen the city walls, that were being undermined by the nearby Skopas river (modern Aladağ Cayı). In late antiquity, the city belonged to the province of Galatia Prima, and later to the Bucellarian Theme.
In ca. 880 the city changed its name again to Basilaion (Βασιλαίον), Basileon (Βασιλέον) or Basileion (Βασίλειον) in honour of the Emperor Basil I (r. 867–886), and a late document of the Byzantine Church refers to it by its original name of Γορδίου Κώμη. Symeon the New Theologian was born in Basileion in 949 AD. Emperor Constantine X Doukas (r. 1059–1067) raised the see to the rank of a metropolis (without suffragans) in order to honour its incumbent bishop, later confirmed by Michael VII Doukas (r. 1071–1078). Although intended as temporary, the rank was retained, despite the protestations of Nicetas, Metropolitan of Ancyra, to Emperor Alexios I Komnenos (r. 1081–1118).
The site of the city has been securely identified thanks to the preservation of milestones with the inscription Iuliopolis, from the Roman road connecting Nicaea with Ancyra. The western parts of the site now lie submerged in the Sarıyar Dam reservoir, as well as the nearby Byzantine-era bridge (Sarılar Köprüsü) over the Skopas river.
A few of the bishops of Juliopolis/Basilaion are known: