Provided is a three-dimensional semiconductor memory device including a peripheral logic structure on a semiconductor substrate to include peripheral logic circuits and a lower insulating gapfill layer, a horizontal semiconductor layer on the peripheral logic structure, stacks on the horizontal semiconductor layer, each of the stacks including a plurality of electrodes vertically stacked on the horizontal semiconductor layer, and a plurality of vertical structures passing through the stacks and connected to the horizontal semiconductor layer. The horizontal semiconductor layer may include a first semiconductor layer disposed on the lower insulating gapfill layer and co-doped with an anti-diffusion material and first conductivity type impurities of a first impurity concentration, and a second semiconductor layer disposed on the first semiconductor layer and doped with first conductivity type impurities of a second impurity concentration lower than the first impurity concentration or undoped.