A semiconductor device having stable electric characteristics is provided. The transistor includes first to third oxide semiconductor layers, a gate electrode, and a gate insulating layer. The second oxide semiconductor layer has a portion positioned between the first and third oxide semiconductor layers. The gate insulating layer has a region in contact with a top surface of the third oxide semiconductor layer. The gate electrode overlaps with a top surface of the portion with the gate insulating layer positioned therebetween. The gate electrode faces a side surface of the portion in a channel width direction with the gate insulating layer positioned therebetween. The second oxide semiconductor layer includes a region having a thickness greater than or equal to 2 nm and less than 8 nm. The length in the channel width direction of the second oxide semiconductor layer is less than 60 nm.