Disclosed is a three-dimensional semiconductor device including a stack structure on a substrate and including electrodes that are vertically stacked on top of each other on a first region of a substrate, a vertical structure penetrating the stack structure and including a first semiconductor pattern, a data storage layer between the first semiconductor pattern and at least one of the electrodes, a transistor on a second region of the substrate, and a first contact coupled to the transistor. The first contact includes a first portion and a second portion on the first portion. Each of the first portion and the second portions has a diameter that increases with an increasing vertical distance from the substrate. A diameter of an upper part of the first portion is greater than a diameter of a lower part of the second portion.