Dam construction, as one of the oldest and most complex construction activities, has always been considered by different societies and is also considered one of the important economic resources of each country and region. Concern about the safety of dams has existed for a long time and is increasing day by day. The design and implementation of dams and their safety against earthquakes are of great importance. The main concept of safety is to prevent the failure of a dam and the uncontrolled overflow of the reservoir water with the occurrence of floods, which usually causes greater economic damage to the dam. To assess the safety and stability of existing dams, to determine the adequacy of the proposed reforms for the improvement and upgrading of old dams, and to evaluate the proposed designs for the construction of new dams, it is necessary to evaluate the effects of earthquakes on these dams. Predicting the behavior of concrete dams during earthquakes is one of the most complex and difficult problems in structural dynamics. The overall objective of this study is to numerically investigate the seismic behavior of a concrete dam with a buttress (Case study: Manjil Dam) using the finite element method. ABAQUS software was used for the study. For this purpose, the dam was modeled in two cases A (Dam without a buttress) and B (Dam with buttress). According to the Nonlinear Static Analysis (NSP) of the pushover, comparing the force-displacement diagram of model A and model B, we observed that placing a buttress in model B increased the resistance by 23.45% compared to model A. Also, according to the results obtained, it was observed that the strain contour created by the NSP in the model B sample caused the Tresca shear stress to decrease by an average of 23.97% in the dam body.