This study aimed at assessing the static and dynamic slope stability of Ribb Zoned Earthen Dam that is located in the earthquake prone areas of Ethiopia by employing finite element method using the shear strength reduction (SSR) technique of PLAXIS. The slope stability analyses were coupled with seepage analyses by considering different critical loading conditions. The peak ground acceleration (PGA) required as an input for the dynamic analyses of the dam were determined from a real accelerogram of an earthquake of magnitude 6.5 Richter scale recorded in 1845 around the dam, at epicenter distance of 175 km. Since the Ribb dam site and reservoir area lie in an earthquake sensitive zone of magnitudes 4.5 to 6.5, additional dynamic analyses have also been carried out corresponding to an earthquake of magnitude 4.5. The results of the coupled analyses using these two values of PGA showed that, the dam is stable at all critical conditions. Deformation analyses also showed that all displacements in the dam crest are within permissible limits.