A vertical semiconductor device including a plurality of vertical memory cells on an upper surface of a first substrate, an adhesive layer on a lower surface of the first substrate that is opposite to the upper surface of the first substrate, a second substrate having first peripheral circuits thereon, a lower insulating interlayer on the second substrate, and a plurality of wiring structures electrically connecting the vertical memory cells and the first peripheral circuits. A lower surface of the adhesive layer and an upper surface of the lower insulating interlayer may be in contact with each other.