A semiconductor device including a stack structure including gate layers and interlayer insulating layers spaced apart in a vertical direction, a channel hole penetrating the stack structure in the vertical direction, a core region extending within the channel hole, a channel layer disposed on a side surface of the core region, a first dielectric layer, a data storage layer and a second dielectric layer, which are disposed between the channel layer and the gate layers, and a pad pattern disposed on the core region, in the channel hole, and in contact with the channel layer. A first horizontal distance between a side surface of a first portion of an uppermost gate layer and an outer side surface of the channel layer is greater than a second horizontal distance between a side surface of a second portion of the uppermost gate layer and an outer side surface of the pad pattern.