This paper intends to re-investigate the so-called church of St. Clement in Ankara, in terms of topography, architecture and art history, within the historical context of Byzantine Ankara. It also aims to discuss and interpret certain aspects of this building, which often remained outside the context of those conventional typological and chronological considerations, and thus attempts to restore the history of this church, and its possible building phases, in the light of archaeological and textual evidence from the 4th to the 9th centuries. Today little can be seen of this building, which was almost completely lost before the 1960s. The remains of this church are hidden (and not easily accessible) behind buildings at the corner of Denizciler Caddesi and Çıkrıkçılar Yokuşu in the historic district of Ulus. Only the inner section of a wall, including a marble block with an incised Latin cross with triangular termini (horizontal arms being eroded), survives. The church of St. Clement, built of alternating rows of rubble stone and brick (pilasters supporting the dome and arches are completely of brick), had a cross-inscribed (cross-in-square) plan (with a central dome), terminating in an apse, with semi-circular interior and polygonal exterior, including a narthex, galleries, and crypt. The date of St. Clement s has long been debated, although no comprehensive research has focused on this building after the first half of the last century. Scholars have proposed different dates, oscillating from the late 5th/early 6th to the middle of the 9th century, on the basis of typological and stylistic considerations. The layout of this church shares similarities to a group of medieval churches, including the variations of the cross-domed plan, such as the Koimesis church in Nicaea/Iznik, the church of St. Nicholas at Myra/Demre and St. Sophia at Bizye/Vize, to cite only a few examples. In terms of construction technique and materials, St. Clements s, built of bands of brick and rubble, finds close parallels in the masonry of the Dereağzı church in Lycia, as well as in the superstructure of the inner circuit of the Ankara citadel, also built of alternating rows of rubble stone and brick and attributed to the year 859. A closer examination of its architectural characteristics and construction technique and materials, together with a re-interpretation of archaeological and literary evidence, strongly indicates a date around the middle of the 9th century for the construction of St. Clement s. On the other hand, the cross-domed church of St. Clement might well have been built on the site of an earlier Christian place (or places) of worship. Indeed, St. Clement s, located at the southwest foot of the citadel hill, is supposed to have been erected at a place called Cryptus, where St. Clement received martyrdom under Diocletian (284-305) and was buried together with one of his deacons. According to Vita S. Clementis Ancyrani, an earlier Christian place of worship was erected in the area where the later church of St. Clement stood. The church of St. Clement also includes a number of marble architectural elements, attributable to the 5th and 6th centuries. It is very probable that these marble architectural elements, showing a significant contrast in terms of decoration and workmanship to the rubble and brick masonry of the walls, had been removed from an earlier Christian place of worship (in all probability that at Cryptus) to be reused in the 9th-century church, as well as in the citadel walls and other locations in Ankara.