Provided are three-dimensional semiconductor memory devices and methods of forming the same. The device includes a substrate, conductive patterns stacked on the substrate, and an active pattern penetrating the conductive patterns to be connected to the substrate. The active pattern may include a first doped region provided in an upper portion of the active pattern, and a diffusion-resistant doped region overlapped with at least a portion of the first doped region. The diffusion-resistant doped region may be a region doped with carbon.