A semiconductor device includes an electrode structure including electrodes stacked on a substrate and an insulating pattern on an uppermost electrode of the electrodes, a vertical structure that penetrates the electrode structure and is connected to the substrate, a first insulating layer on the electrode and the vertical structure, a conductive pattern that penetrates the first insulating layer and is connected to the vertical structure, an upper horizontal electrode on the conductive pattern, and an upper semiconductor pattern that penetrates the upper horizontal electrode and is connected to the conductive pattern. The conductive pattern has a first side surface on the vertical structure and a second side surface on the insulating pattern.