An optical semiconductor device according to the present disclosure includes a semiconductor substrate, at least one semiconductor laser provided on the semiconductor substrate, an optical multiplexer/demultiplexer circuit provided on the semiconductor substrate, multiplexing or demultiplexing first output light of the semiconductor laser and outputting second output light and third output light, a first waveguide portion provided on the semiconductor substrate, and outputting the second output light from an end face of the semiconductor substrate, and a second waveguide portion including an optical amplifier amplifying the third output light and a reflecting portion and provided on the semiconductor substrate, wherein the reflecting portion includes a diffraction grating that reflects the third output light amplified by the optical amplifier to feed back to the semiconductor laser via the optical amplifier and the optical multiplexer/demultiplexer circuit, and the semiconductor laser and the reflecting portion form a resonator.