The seismic performance of the tallest roller-compacted concrete gravity dam in Turkey is investigated in this study. For this purpose, two- and three-dimensional nonlinear dynamic finite element analyses were conducted to investigate the expected damage under simulated ground motions. Based on the results of finite element simulations, it was found that the three-dimensional behaviour of the dam is substantially different from the idealised two-dimensional monolith behaviour, even for a dam having monoliths separated with transverse joints. In addition, it was determined that in order to limit the cracking at the base of the dam, higher strength grout-enriched concrete must be used on the upstream and downstream faces. This study demonstrates the importance of three-dimensional modelling for gravity dams built in relatively narrow valleys along with the significance of the modelling of the nonlinear response for seismic performance assessment.