This paper presents a study for assessing the seismic performance of the Bouçã dam (Portugal), a thin double-curvature arch dam, standing 63 m high, with a 175 m long crest. In operation since 1955, the dam features a horizontal crack, located about 25 m below the crest, which extends from the central section towards the abutment of the right bank [1]. The numerical computations are carried out using the 3D finite element program DamDySSA, developed by the authors for dynamic analysis of dam-foundation-reservoir systems [2]. In this program, the non-linear seismic response is computed using a time-stepping technique combined with an iterative stress-transfer method, adopting (a) a joint constitutive model to simulate the structural effects due to the opening/closing and sliding movements of joints and cracks, and (b) a concrete constitutive damage model with softening with two independent damage variables, to simulate the material damage under tension and compression. The non-linear seismic behaviour of the Bouçã dam is analysed in this work for a load combination including the dam self-weight, the hydrostatic pressure of the reservoir and a generated seismic action of increasing intensity. First, the structural effects due to the movements of the vertical contraction joints and especially of the horizontal crack on the structural response of the dam are evaluated, particularly in what concerns the deformations and stress fields. Subsequently, the seismic safety of the Bouçã dam is assessed for both the Operating Basis Earthquake (OBE) and the Maximum Design Earthquake (MDE), using a method [3] based on Endurance Time Analysis (ETA) in which the seismic performance of the dam is evaluated by controlling the evolution of the tensile and compressive damage states.