According to one embodiment, a semiconductor device includes first, second, and third electrodes, and first, second, and third semiconductor regions. The third electrode is between the first electrode and the second electrodes. The first semiconductor region includes Alx1Ga1-x1N and includes first to seventh partial regions. The fourth partial region is between the first partial region and the third partial region. The fifth partial region is between the third partial region and the second partial region. The second semiconductor region includes Alx2Ga1-x2N and includes first and second semiconductor portions. The sixth partial region is between the fourth partial region and the first semiconductor portion. The seventh partial region is between the fifth partial region and the second semiconductor portion. The third semiconductor region includes Alx3Ga1-x3N and includes a first semiconductor film part. The first semiconductor film part is between the sixth partial region and the third electrode.