At postglacial rebound time scales, the intraplate continental lithosphere typically behaves as an elastic solid. However, under exceptional conditions, the effective viscosity of the lower crust and lithospheric mantle may be as low as ∼10²⁰ Pa s, leading to ductile behavior at postglacial rebound time scales. We studied the effects of a lithospheric ductile zone on postglacial rebound–induced seismicity and deformation in eastern Canada and the northeastern United States using three types of models: (1) a reference model with no lithospheric ductile layer; (2) a model with a uniform, 25-km-thick, ductile layer embedded in the middle of the lithospheric column; and (3) a model with a dike-like vertical ductile zone, extending from mid-crust level down to the bottom of the lithosphere, along the Precambrian rift structure of the St. Lawrence Valley. Based on geothermal and rock physics data, the viscosity of the ductile zone is set to either 10²⁰ or 10²¹ Pa s. We found that a narrow ductile zone cutting vertically through the lithosphere has larger effects than the uniformly thick horizontal ductile layer. Effects of a lithospheric weak zone on uplift rates may be large enough to be detected by global positioning system (GPS) measurements, especially for low viscosities. While the effect on fault stability is also large, the impact on the onset time of instability is small for sites within the ice margin. The impact on the onset time is more significant for sites outside the ice margin. Effects of a lithospheric weak zone are also significant on present-day horizontal velocities and strain rates and are at the limit of resolution for GPS measurements.