The effect of three-dimensional geometry on the seismic response of open-top rectangular concrete water tanks is investigated. In this study, the fluid-structure interaction is introduced incorporating wall flexibility. Numerical studies are done based on finite element simulation of the tank-liquid system. The ANSYS finite element program is used. The liquid-tank system is modelled assuming both 2D and 3D geometries. Parametric studies are conducted to investigate the effect of water level, tank plan dimensions and the nature of the ground motion on the dynamic response. Due to three-dimensional geometry, amplification of the dynamic response in the form of sloshing height, hydrodynamic pressures and resultant forces is observed. The results show that, at the corner of the tanks, the interaction of the waves generated in longitudinal and transverse directions initiates greater wave amplitude. Sensitivity of the sloshing response of the tank to the frequency content of the ground motion is observed.