A three-dimensional semiconductor device includes bit lines provided on a substrate, a gate structure provided between the substrate and the bit lines, a common source line provided between the gate structure and the bit lines, and channel pipes connecting the bit lines to the common source line. Each of the channel pipes may include a pair of vertical portions extending through the gate structure and a horizontal portion connecting the vertical portions. The pair of vertical portions are provided under a pair of the bit lines arranged adjacent to each other, respectively.