A three-dimensional semiconductor memory device includes a substrate including cell and connection regions. An electrode structure is disposed on the substrate, the electrode structure having a staircase structure on the connection region. A first vertical channel structure and a first dummy structure at least partially penetrate the electrode structure on the cell region and the connection region, respectively. Bottoms of expanded portions of the first vertical channel structure and the first dummy structure are located at first and second levels, respectively. The second level is higher than the first level.