Prusias ad Hypium was a city in ancient Bithynia, and afterwards in the late Roman province of Honorias, and a bishopric that was a suffragan of Claudiopolis in Honoriade. Before its conquest by King Prusias I of Bithynia it was named Cierus or Kieros ( Ancient Greek: Κίερος). Photius writes that it was called Kieros, from the river which flows by it.
It site was near Konuralp,
Düzce Province, Turkey. 
Memnon of Heraclea says that King Prusias I of Bithynia (237-192 B.C.) captured from the Heracleans the town of Kieros, united it to his dominions and changed its name to Prusias.  Pliny and  Ptolemy merely mention it, one below Mt. Hypius, the other near the River  Hypius.
Several of its bishops are known:
George (not Hesychius, as
Le Quien says), 325; Olympus in 451;
Dometius in 681;
Theophilus in 787;
Constantine in 869;
Leo in 879;
St. Paul, martyred by the iconoclasts in the ninth century. 
It is not known when this see disappeared; it still existed in the tenth century.
No longer the seat of a residential bishop, it remains a
titular see of the Roman Catholic Church.
Photius, Bibliotheca excerpts, § 224.32.1
Richard Talbert, ed. (2000). . Princeton University Press. p. 86, and directory notes accompanying. Barrington Atlas of the Greek and Roman World
Lund University. . Digital Atlas of the Roman Empire
^ "Frag. histor. Graec.", coll. Didot, frag. 27 and 47; fragment 41 treats of Kios/
Cius or Guemlek, also called Prusias, and not of Kieros, as the copyist has written; this has given rise to numerous confusions.
Pliny. . Naturalis Historia 5.43.
Ptolemy. . 5.1.13. The Geography
Le Quien, Oriens christianus I, 579.
Heinrich Gelzer, Ungedruckte ... Texte der Notitiæ episcopatuum, 554.
This article incorporates text from a publication now in the public domain: Smith, William, ed. (1854–1857). "Prusa". . London: John Murray. Dictionary of Greek and Roman Geography
This article incorporates text from a publication now in the public domain: Herbermann, Charles, ed. (1913). . . New York: Robert Appleton. Catholic Encyclopedia The entry cites:
DE HELL, Voyage en Turquie et Perse, IV, 334-38, 353-73;
TEXIER, Asie Mineure, 85;
LE BAR, Voyage archéologique, 1174–82;
PERROT, Expédition archéologique de la Galatie et de la Bithynie (Paris, 1872, 20-42).