A mid-IR TDLAS sensor for NO measurements in high-temperature environments is presented. The diagnostic targets the strong NO transition at 1977.27 cm−1, which provides low spectral interference and weak temperature sensitivity, thus reducing the effect of temperature inhomogeneity along the optical path on the retrieved path-averaged NO concentration. The diagnostic was validated in plasma-heated gases with H2O vapor and in pilot-scale biomass/hydrogen combustion at temperatures up to 1450 K and H2O mole fractions up to 30%. NO concentration obtained by fitting HITEMP-based spectra agreed with FTIR measurements within 15%, with estimated uncertainties of 20%. The sensor achieved a detection limit of about 50 ppm·m. Neighboring H2O transitions were negligible at lab-scale but required in pilot-scale fitting