A tunable distributed feedback semiconductor laser includes a substrate; an optical waveguide structure disposed on a main surface of the substrate and including an active layer and a diffraction grating, the optical waveguide structure being divided into a first DFB portion, a wavelength-tuning region, and a second DFB portion in that order; a first electrode for injecting carriers into the active layer in the first DFB portion; a second electrode for injecting carriers into the active layer in the second DFB portion; and a third electrode for supplying a wavelength tuning signal to the wavelength-tuning region. The diffraction grating extends over the first DFB portion, the wavelength-tuning region, and the second DFB portion. An optical confinement factor of the wavelength-tuning region is smaller than that of the first and second DFB portions.