The understanding of the dynamic behavior of an arch dam and its evolution over time provides important data for safety control procedures. Forced vibration tests remain a reliable technique for dynamic characterization, providing the response to excitation levels higher than ambient vibration measurements. Forced vibration tests were performed before and after the reservoir filling in a recent arch dam built in Portugal. Numerical models of the dam-reservoir-foundation system were built and calibrated with the experimental data. A good agreement was achieved between the numerical simulations and experimental data for the five lowest frequencies and their corresponding mode shapes.