A three-dimensional semiconductor memory device is disclosed. The device may include a first source conductive pattern comprising a polycrystalline material including first crystal grains on a substrate, the substrate may comprising a polycrystalline material including second crystal grains, a grain size of the first crystal grains being smaller than a grain size of the second crystal grains, a stack including a plurality of gate electrodes, the plurality of gates stacked on the first source conductive pattern, and a vertical channel portion penetrating the stack and the first source conductive pattern, and the vertical channel portion being in contact with a side surface of the first source conductive pattern.