An optical semiconductor device includes a semiconductor substrate with a protrusion that forms a lower end portion of a mesa stripe structure in a stripe shape extending in a first direction; a multi-quantum well layer in a stripe shape extending in the first direction on the protrusion, wherein the multi-quantum well layer forms an intermediate portion of the mesa stripe structure; a semiconductor layer in a stripe shape extending in the first direction on the intermediate portion, wherein the semiconductor layer forms an upper end portion of the mesa stripe structure; and a semi-insulating semiconductor layer in contact with side surfaces of the mesa stripe structure on both sides in a second direction perpendicular to the first direction. The optical semiconductor device may include a first electrode on a surface of the semiconductor substrate and/or a second electrode on the upper end surface of the mesa stripe structure.