A three-dimensional (3D) semiconductor device includes a stack structure including electrodes vertically stacked on a substrate, a channel structure coupled to the electrodes to constitute a plurality of memory cells three-dimensionally arranged on the substrate, the channel structure including first vertical channels and second vertical channels penetrating the stack structure and a first horizontal channel disposed under the stack structure to laterally connect the first vertical channels and the second vertical channels to each other, a second horizontal channel having a first conductivity type and connected to a sidewall of the first horizontal channel of the channel structure, and conductive plugs having a second conductivity type and disposed on top ends of the second vertical channels.