An interposer and a semiconductor device including the interposer, which can prevent thermal warpage of an insulative substrate. The interposer is provided with a semiconductor chip in a semiconductor device and may be disposed between the semiconductor chip and a mount board. The interposer includes: a substrate of an insulative resin; an island on one surface of the substrate to be bonded to a rear surface of the chip; a thermal pad on the other surface opposite the one surface opposed to the island with the intervention of the substrate; and a thermal via extending through the substrate from the one surface to the other surface to thermally connect the island to the thermal pad.