An environmentally friendly analog of the prominent methylammonium lead halide perovskite, methylammonium bismuth bromide (MA3Bi2Br9), was prepared and investigated in the form of powder, single crystals and nanowires. Complete characterization via synchrotron X-ray diffraction data showed that the bulk crystal does not incorporate water into the structure. At the same time, water is absorbed on the surface of the crystal, and this modification leads to the changes in the resistivity of the material, thus making MA3Bi2Br9 an excellent candidate for use as a humidity sensor. The novel sensor was prepared from powder-pressed pellets with attached carbon electrodes and was characterized by being able to detect relative humidity over the full range (0.7–96% RH) at ambient temperature. Compared to commercial and literature values, the response and recovery times are very fast (down to 1.5 s/1.5 s).