As a new IV–V group semiconductor, germanium‐diarsenide (GeAs₂) compounds have attracted considerable attention due to their outstanding optical and electrical properties, thickness‐dependent bandgap, in‐plane anisotropy, and excellent optical absorption. However, the potential of GeAs₂ in the field of ultrafast and ultranarrow fiber laser has not been studied. In this article, a high‐quality GeAs₂ nanosheets saturable absorber (SA) is successfully prepared by liquid‐phase exfoliation. The nonlinear optical characteristics of GeAs₂ nanosheets have been investigated based on a balanced twin‐detector measurement system. The modulation depth, nonsaturable loss, and saturation intensity are measured to be 5.2%, 24%, and 1.23 GW cm⁻², respectively. GeAs₂ has been successfully applied as an SA in an ultrafast and single‐frequency fiber laser. A stable mode‐locked laser pulses operation with a duration as short as 371 fs and a repetition rate of 8.19 MHz at a wavelength of 1560 nm is achieved. Moreover, ultranarrow fiber lasers with a high signal‐to‐noise ratio of 80 dB and a linewidth of ≈678 Hz are obtained. The findings validate that 2D GeAs₂ can be used as an SA and has promising applications in ultrafast and ultranarrow photonics.