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In this paper, we report an all-optical NH₃ gas sensor based on graphene/microfiber hybrid waveguide (GMHW). The study on the sensing mechanism shows that as the adsorption of NH₃ modifies the conductivity of graphene and thus the effective refractive index of the GMHW, and the transmitting light along the GMHW is very sensitive to NH₃ gas concentration. The wavelength shift induced by the NH₃ absorption is spectrally demodulated by using a microfiber-based Mach–Zehnder interferometer (MZI). A high sensitivity of ∼6 pm/ppm is obtained for the NH₃ adsorption measurement. The resolution of such a sensor is ∼0.3 ppm, mainly limited by the resolution of the optical spectrum analyzer used. The work of this paper may open a window for the development of novel GMHW-based gas sensors with high sensitivity, small footprint, easy fabrication and low cost.