Seismic sliding of rock masses is a serious earthquake hazard. Under the action of earthquake loads, rock blocks may move longitudinally downwards inclined surfaces, while the lateral movement also could be significant. In the present work, the seismic movements of rock blocks in the three-dimensional space is studied. On the one hand, theoretical solutions of a single block moving on an incline under seismic loads is derived. On the other hand, the seismic loading function is added to the three-dimensional discontinuous deformation analysis (3D DDA) program that was enhanced by the Contact Theory for block contact detection, and 3D DDA simulation of seismic movements of rock blocks on inclines is carried out. It shows that the theoretical and numerical results of the acceleration, velocity and displacement in the longitudinal and lateral directions of a single block on an incline agree well under various seismic loads parameters (amplitude, phase and frequency) and incline parameters (slope and friction coefficient). Influences of the seismic loads in the three directions (one in vertical and two in horizontal) on the longitudinal and lateral movements of a single block and multiple rock blocks are carefully studied through the theoretical and numerical analysis. This work builds good basis for the further 3D DDA simulation of earthquake-induced rock mass movements and the further investigations of seismic sliding hazard.