Passively Q-switched ytterbium-doped fiber lasers (YDFLs) incorporating a molybdenum sulfide selenide (MoSSe)-based saturable absorber (SA) are demonstrated. The modulation depth and saturation intensity of the MoSSe-based SA are measured to be approximately 25.0% and 0.002 MW/cm2, respectively, using the twin detector technique. The YDFL’s output has a center wavelength of 1038.5 nm with a top pulse width and energy of 1.2 μs and 18.9 nJ, respectively, at a pump power of 333 mW. The MoSSe-based SA has a good linear optical response, providing significant opportunity for use in applications over an ultra-broadband spectrum, particularly spectroscopy and biomedical diagnostics.