A transistor with high and stable electrical characteristics whose parasitic capacitance is suppressed is provided even when miniaturized. A semiconductor device including it also achieves higher performance and reliability. The transistor includes an oxide semiconductor over a substrate, first and second electrodes in contact with the oxide semiconductor, an insulator over the oxide semiconductor, and the first and second electrodes, and a third electrode over the insulator. The oxide semiconductor includes a first region, a second region, and a third region overlapping with the first electrode, the second electrode, and the third electrode, respectively. The first region and the second region include a fourth region and a fifth region each overlapping with the third region, respectively. Top surfaces of the first and second electrodes are provided on the same plane as a top surface of the oxide semiconductor or the side closer to the oxide semiconductor than the same plane.