Disclosed are three-dimensional semiconductor memory devices and methods of fabricating the same. A three-dimensional semiconductor memory device including a substrate including a cell array region and a connection region, an electrode structure including a plurality of electrodes and a plurality of dielectric layers alternately stacked on the substrate, the electrode structure having a stepwise portion on the connection region, an etch stop structure on the stepwise portion of the electrode structure, and a plurality of contact plugs on the connection region, the contact plugs penetrating the etch stop structure and connected to corresponding pad portions of the electrodes, respectively, may be provided. The etch stop structure may include an etch stop pattern and a horizontal dielectric layer, which has have a uniform thickness and covers a top surface and a bottom surface of an etch stop pattern.