A three-dimensional (3D) semiconductor device includes a stack structure including first and second stacks stacked on a substrate. Each of the first and second stacks includes a first electrode and a second electrode on the first electrode. A sidewall of the second electrode of the first stack is horizontally spaced apart from a sidewall of the second electrode of the second stack by a first distance. A sidewall of the first electrode is horizontally spaced apart from the sidewall of the second electrode by a second distance in each of the first and second stacks. The second distance is smaller than a half of the first distance.