In this work, MXene/NiO-composite-based formaldehyde (HCHO) sensing materials were successfully synthesized by an in situ precipitation method. The heterostructures between the MXene and NiO nanoparticles were verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The HCHO sensing performance of the MXene/NiO-based chemiresistive-type sensors was investigated. Compared to pure MXene and NiO materials, the sensing performance of the MXene/NiO-P2-based sensor to HCHO gas at room temperature was significantly enhanced by the formation of MXene/NiO heterojunctions. The response of the MXene/NiO-P2 sensor to 50 ppm HCHO gas was 8.8, which was much higher than that of the pure MXene and NiO. At room temperature, the detectable HCHO concentration of the MXene/NiO-P2-based sensor was 1 ppm, and the response and recovery time to 2 ppm HCHO was 279 s and 346 s, respectively. The MXene/NiO-P2 sensor also exhibited a good selectivity and a long-term stability to HCHO gas for 56 days. The in situ Fourier transform infrared (FTIR) spectra of the MXene/NiO-P2 sensor, when exposed to HCHO gas at different times, were investigated to verify the adsorption reaction products of HCHO molecules.