An optical semiconductor element includes a substrate and a plurality of cells. Each cell includes an optical layer, a first semiconductor layer, and a second semiconductor layer. The plurality of cells include a first cell and a second cell. The second semiconductor layer of the first cell and the first semiconductor layer of the second cell are electrically connected to each other by a first connection portion of a first wiring portion. The first wiring portion has a first extending portion that extends from the first connection portion so as to surround four side portions of the optical layer of the first cell. The optical layer is an active layer that generates light having a central wavelength of 3 μm or more and 10 μm or less or an absorption layer having a maximum sensitivity wavelength of 3 μm or more and 10 μm or less.