A thin film transistor substrate includes a substrate; a first thin film transistor on the substrate and including a polycrystalline semiconductor layer, a first gate electrode on the polycrystalline semiconductor layer, a first source electrode, and a first drain electrode; a second thin film transistor on the substrate and including a second gate electrode, an oxide semiconductor layer on the second gate electrode, a second source electrode, and a second drain electrode; an intermediate insulating layer on the first gate electrode and the second gate electrode and under the oxide semiconductor layer; and a dummy layer between the first source electrode and the intermediate insulating layer and between the first drain electrode and the intermediate insulating layer, wherein the dummy layer is formed of a same material as the oxide semiconductor layer.