Semiconductor device and data storage system including the same

A semiconductor includes a lower structure and a stack structure having interlayer insulating layers and horizontal layers alternately stacked on the lower structure. A first dam vertical structure penetrates the stack structure. The first dam vertical structure divides the stack structure into a gate stack region and an insulator stack region. The horizontal layers include gate horizontal layers in the gate stack region and insulating horizontal layers in the insulator stack region. A memory vertical structure and a supporter vertical structure penetrate the gate stack region. Separation structures penetrate the gate stack region. One separation structure includes a first side surface, a second side surface not perpendicular to the first side surface, and a connection side surface extending from the first side surface to the second side surface. The connection side surface is higher than an uppermost gate horizontal layer of the gate horizontal layers.