Plagioclase feldspar is the most abundant rock‐forming mineral in the crust of the Earth, Moon, and Mars and is also an important component in some minor bodies in the Solar System. The distribution, abundance, and precise composition of plagioclase on planetary surfaces from remote sensing data are important measurements for evaluating changing conditions during magma evolution. Optical constants are critical input parameters in radiative transfer theory, which enables modeling of spectra for the extraction of mineral abundances and grain sizes from a remotely sensed spectrum. Mid‐infrared (MIR) optical constants of most triclinic rock‐forming minerals are not available due to the complexity associated with the derivation of optical constants of low‐symmetry minerals. In this work, we have calculated the MIR optical constants of a labradorite single crystal using dispersion theory and laboratory reflectance spectra at non‐normal incidence. The optical constants we derived here will assist in modeling spectra in the MIR region and quantifying mineral composition, particle size, and abundances from remote sensing data.