This paper presents a study of the seismic behaviour of a 70 m high double-curvature arch dam with a horizontal crack, using a 3D finite element (FE) model of the dam-reservoir-foundation system. The analysis was conducted with DamDySSA5.0, a program developed at LNEC by the authors, which incorporates the non-linear behaviour of discontinuities, including vertical contraction joints and a horizontal crack, under three different water level values. Concrete failure is modelled using a damage-based approach with two independent variables for tension and compression. The simulations were performed on models with and without a horizontal crack, located 30 m below the crest. A non-linear seismic analysis was carried out, considering the crack’s effects (with or without the horizontal crack), three different water level conditions, and a seismic accelerogram with a peak ground acceleration (PGA) of 0.3g. The model enabled the assessment of: (i) the horizontal crack behaviour, and (ii) the influence of the water level on the dam’s non-linear response. Results highlight the significant impact of dam body discontinuities and water level variations on the seismic performance of the dam-foundation-reservoir system.