A highly reliable semiconductor device with favorable electrical characteristics is provided. A semiconductor device includes a semiconductor layer, an insulating layer, a metal oxide layer, and a conductive layer. The semiconductor layer, the insulating layer, the metal oxide layer, and the conductive layer are stacked in this order. The semiconductor layer includes a first region, a pair of second regions, and a pair of third regions. The first region overlaps the metal oxide layer. The second regions sandwich the first region, overlap the insulating layer, and do not overlap the metal oxide layer. The third regions sandwich the first region and the pair of second regions, and do not overlap the insulating layer. The third region includes a portion having a lower resistance than the first region. The second region includes a portion having a higher resistance than the third region.