The number of strong ground motion recordings available for regression analysis in developing empirical attenuation relationships has rapidly grown in the last 10 years. However, the dearth of strong-motion data from the Cascadia subduction zone has limited this development of relationships for the Cascadia subduction zone megathrust, which can be used in the calculation of design spectra for engineered structures. A stochastic finite-fault ground-motion model has been used to simulate ground motions for moment magnitude (M) 8.0, 8.5, and 9.0 megathrust earthquakes along the Cascadia subduction zone for both rock-and soil-site conditions. The stochastic finite-fault model was validated against the 1985 M 8.0 Michoacan, Mexico, and the 1985 M 8.0 Valpariso, Chile, earthquakes. These two subduction zone megathrust earthquakes were recorded at several rock sites located near the fault rupture. For the Cascadia megathrust earthquakes, three different rupture geometries were used to model the M 8.0, 8.5, and 9.0 events. The geometries only differ in their respective fault lengths. A fault dip of 9° to the east with a rupture width of 90 km was selected to represent average properties of the Cascadia subduction zone geometry. A regional crustal damping and velocity model was used with the stochastic finite-fault model simulations. Ground motions were computed for 16 site locations. The parametric uncertainties associated with the variation in source, path, and site effects were included in the development of the ground motions. A functional form was fit to the ground-motion model simulations to develop region-specific attenuation relationships for the Cascadia megathrust rupture zone for both rock and soil site conditions. The total uncertainty was based on a combination of the modeling and parametric uncertainties (sigmas). These newly developed attenuation relationships for Cascadia subduction zone megathrust earthquakes can be used in both the probabilistic and deterministic seismic-hazard studies for engineering design for the Pacific Northwest.