The Cistern of Modestus, located in Region XI near the Church of the Holy Apostles (present-day Fatih Camii complex), is likely the earliest attested cistern in Constantinople. Its altitude suggests it was supplied by the Valens Waterway. While Janin classifies it as a covered cistern, Altuğ describes it as an open cistern, with no surviving remains today.
According to the Patria, Modestus—a patrician brought from Rome by Constantine—built his house and a cistern nearby, confirmed by the Notitia (early 5th century). Crow et al. suggest it may also be the “Cistern of the Archangel” listed in the Codex Matritensis Graecus. By the mid-16th century, Gilles noted that the cistern had disappeared, replaced by the Çukur Hamamı baths built by Mehmed II.
Archaeologically, it may correspond to a large cistern (D5/4, c. 154 × 90 m, 14,000 m²) near Bozdoğan Kemeri and Sehzadebaşı Caddesi, though other nearby cisterns, like the unlocated Arcadiaca, complicate identification. This possible location was noted by Gilles and analysed by Forchheimer and Strzygowski (1894) were some ruins in Saraçhane, but these are now lost.