A three-dimensional semiconductor memory device includes a substrate, an electrode structure including gate electrodes sequentially stacked on the substrate, a source structure between the electrode structure and the substrate, vertical semiconductor patterns passing through the electrode structure and the source structure, a data storage pattern between each of the vertical semiconductor patterns and the electrode structure, and a common source pattern between the source structure and the substrate. The common source pattern has a lower resistivity than the source structure and is connected to the vertical semiconductor patterns through the source structure.