In this paper, we introduce DimensionX, a framework designed to generate photorealistic 3D and 4D scenes from just a single image with video diffusion. Our approach begins with the insight that both the spatial structure of a 3D scene and the temporal evolution of a 4D scene can be effectively represented through sequences of video frames. While recent video diffusion models have shown remarkable success in producing vivid visuals, they face limitations in directly recovering 3D/4D scenes due to poor spatial and temporal controllability during generation. To overcome this difficulty, we propose ST-Director, which decouples spatial and temporal factors in video diffusion by learning dimension-aware directors from dimension-variant data. This decoupled video diffusion enables precise manipulation of spatial structures and temporal dynamics, allowing us to reconstruct both 3D and 4D representations from sequential frames by combining spatial and temporal dimensions. Additionally, to bridge the gap between generated videos and real-world scenes, we introduce a trajectory-aware mechanism for 3D generation and an identity-preserving denoising strategy for 4D generation, respectively. Extensive experiments on various real-world and synthetic datasets demonstrate that DimensionX achieves state-of-the-art performance in decoupled video generation, as well as 3D and 4D scene generation.