The characterization of the dynamic behavior of an arch dam, and its evolution throughout the structure’s lifetime, is an important component of the safety control process. Forced vibration tests remain a reliable technique for this purpose. The Baixo Sabor dam is a 123 m high arch dam recently built in Portugal which is presently undergoing the first filling. Forced vibration tests were performed before the reservoir filling. The test methodology and results are described, and the dynamic behavior is analyzed by means of a numerical model.