Three-dimensional (3D) semiconductor memory devices and methods of manufacturing the same are provided. Three-dimensional (3D) semiconductor memory devices may include a substrate including a cell array region and a connection region, a lower stack structure including a plurality of lower electrodes vertically stacked on the substrate, the lower stack structure having a first stair step structure extending in a first direction on the connection region and a second stair step structure extending in a second direction substantially perpendicular to the first direction on the connection region, and a plurality of intermediate stack structures vertically stacked on the lower stack structure. Each of the intermediate stack structures includes a plurality of intermediate electrodes vertically stacked and has a third stair step structure extending in the second direction on the connection region. Each of the intermediate stack structures exposes the third stair step structure of the intermediate stack structure disposed thereunder.