Abstract: A watertight manhole insert which is characterized by a dish-shaped seal or stopper designed to prevent rainwater from entering a sanitary sewer system through a manhole closed by a manhole cover. The manhole insert is located beneath the manhole cover and serves to plug the manhole and prevent flooding of the underlying sewer system. The manhole insert is characterized by a dish-shaped bowl having a pressure-relief valve, an optional vacuum relief valve and left strap and a substantially perpendicular, upward-standing side wall fitted with an outwardly-extending top rim for engaging a shoulder shaped in a manhole to support the manhole insert. Multiple, upward-standing bottom ribs radiate from the side wall inwardly in the curved bowl bottom toward the pressure relief valve, which is normally located in the center of the manhole insert, in order to stiffen the bowl-shaped bottom. Optional side ribs may also be provided in the vertical side wall for stiffening purposes.