Alloyed ternary CdS_1−XSe_X nanoribbons of variable composition X were synthesized by the combination of thermal evaporation and laser ablation. High-resolution transmission electron microscopy and x-ray diffraction showed that the ternary CdS_1−XSe_X nanoribbons were single phase and highly crystalline. Room-temperature optical measurements showed that band-gap engineering could be realized in CdS_1−XSe_X nanoribbons via modulation in composition X. Lasing emission between the band-gap energy of CdS (512 nm) and that of CdSe (710 nm) was observed for composition 0<X<1 under optical pumping (266 nm) at power densities of 35–60 kW cm⁻². Cathodoluminescence imaging and spectroscopy of single CdS_1−XSe_X nanoribbons reveal the uniform optical properties of the nanoribbons, which supports the absence of phase segregation within the nanoribbon. Fine tuning of the lasing wavelength via composition changes is shown to be smaller than 0.1 nm, and is capable of overlapping thermally induced tuning, demonstrating the possibility of continuous tuning in the lasing wavelength. The broad and fine tunable lasing properties of ternary nanoribbons have potential applications in color-tuned nanolasers, biological labels, and nano-optoelectronics.