A three-dimensional semiconductor device includes: a peripheral circuit structure disposed on a lower substrate, and including an internal peripheral pad portion; an upper substrate disposed on the peripheral circuit structure; a stack structure disposed on the upper substrate, and including gate horizontal patterns; a vertical channel structure passing through the stack structure in a first region on the upper substrate; a first vertical support structure passing through the stack structure in a second region on the upper substrate; and an internal peripheral contact structure passing through the stack structure and the upper substrate, and electrically connected to the internal peripheral pad portion, wherein an upper surface of the first vertical support structure is disposed on a different level from an upper surface of the vertical channel structure, and is coplanar with an upper surface of the internal peripheral contact structure.