A semiconductor device includes a substrate, a semiconductor layer stacked body, and a source electrode and a drain electrode formed on the semiconductor layer stacked body. The semiconductor layer stacked body includes a first nitride semiconductor layer formed on the substrate, and a second nitride semiconductor layer formed on the first nitride semiconductor layer. The semiconductor device further includes a third nitride semiconductor layer formed on the second nitride semiconductor layer and disposed between the source electrode and the drain electrode, and a gate electrode formed on the third nitride semiconductor layer. The semiconductor device includes a first magnesium-containing region having a magnesium concentration of 1×1018 cm−3 or more that is provided right under the third nitride semiconductor layer, from an upper surface of the second nitride semiconductor layer to a position lower than an interface between the first nitride semiconductor layer and the second nitride semiconductor layer.