A semiconductor device of an embodiment includes a substrate, a first electrode, a second electrode, the first electrode provided between the substrate and the second electrode, the oxide semiconductor layer in contact with the first electrode, an oxide semiconductor layer between the first electrode and the second electrode, the oxide semiconductor layer contains Zn and at least one first element selected from In, Ga, Si, Al, and Sn; a conductive layer between the oxide semiconductor layer and the second electrode, the conductive layer in contact with the second electrode, the conductive layer contains O and at least one second element selected from the group consisting of In, Ga, Si, Al, Sn, Zn, and Ti, a gate electrode; and a gate insulating layer between the oxide semiconductor layer and the gate electrode.