The former Church of St. John the Forerunner (Hagios Ioannis Prodromos en to Troullo), now Hırami Ahmet Paşa Mescidi, is located in the northwest part of the city on the fifth hill, near the former Pammakaristos Monastery (now Fethiye Mosque). It is a cross-in-square church with a tripartite bema and a narthex. The church has long been identified with Hagios Ioannes Prodromos en to Troullo and dated to the twelfth century. However the identification and the date are not secure. 
When the Church of Pammakaristos was made the patriarchal seat in 1456, the Patriarch Gennadius had transferred the nuns previously accommodated there to this building. Ahmed Pasha converted the building into a mosque in the 16th century. In the 1960s, the building was the subject of a heavy-handed restoration, during which large parts of the exterior masonry were rebuilt. The interior was renovated in 2011.
It has also suggested that it dates to the ninth century due to the circular unadorned apses and the circular drum of the dome, which are used in the provinces in the ninth and tenth centuries but not found in the capital. In addition, the alternating bands of ashlar and brick in this building typical of Constantinopolitan construction date from the eighth to the tenth centuries, but not thereafter. The absence of recessed-brick masonry favors this date. 
Its eastern end is unusual. All three apses of the bema are semicircular inside and out, a feature uncommon in Constantinople (the other example is Sekbanbaşı). Moreover, the main apse does not communicate with the side rooms. However, the bema likely extended west to include the three eastern bays of the naos. The primary function of the building was funerary. The narthex has at least four arcosolia for burials. Two more arcosolia were located in the naos, each in the middle of the north and south walls.