A semiconductor device includes a first horizontal molding pattern, a horizontal electrode pattern disposed on the first horizontal molding pattern, and a second horizontal molding pattern disposed on the horizontal electrode pattern. A vertical structure extends through the horizontal patterns. The vertical structure includes a vertical electrode pattern, a data storage pattern interposed between the vertical electrode pattern and the horizontal patterns, a first buffer pattern interposed between the data storage pattern and the first molding pattern, and a second buffer pattern interposed between the data storage pattern and the second molding pattern and spaced apart from the first buffer pattern.