In this work, thermal conductivity and anharmonic properties of chemical vapor transport (CVT)‐grown pyrite‐FeS2 and mineral single crystals have been investigated and compared. It has been shown that optothermal Raman technique is able to capture the large thermal conductivity difference between the CVT and mineral samples at low temperatures. This difference is attributed to point defects such as sulfur vacancies or impurity doping of CVT‐grown FeS2 crystals. Balkanski–Klemens model analysis of the samples has shown that three‐phonon scattering of Ag and Eg modes and the lattice thermal expansion are the dominant anharmonic contributors while four‐phonon scattering is negligible in pyrite‐FeS2. Thus, thermal conductivity of materials that is difficult to measure by conventional methods (i.e., flash method) can be measured in their most native form by using optothermal Raman spectroscopy (RS) without rigorous sample preparation.