The Mocius Reservoir (Region XII) is the largest open cistern, measuring 170 × 147 × 12 meters with a capacity of about 299,880 m³. Due to water pressure, it could only be safely filled to 9.5 meters. Located at 55 meters altitude on the VIIth hill (Xerolophos), it was not directly served by the main Hadrian or Valens waterways, but may have been supplied by a branch of the Thracian line, as suggested by the 10th-century Book of Ceremonies. The area’s limited water demand is reflected by the presence of only three other cisterns nearby.

The reservoir was likely named after St. Mokios, whose church stood close by. Its location between the Constantinian and Theodosian walls, like the Aetius and Aspar reservoirs, suggests agricultural use. However, its brickwork differs from typical 5th-century techniques, resembling instead the Anastasian wall, indicating a later construction date. An Arab account from c. 910 implies the upper Valens line supplied the Mocius reservoir. Today, the site is Findikzade Çukurbostan Park.