AbstractAll spectrometers rely on some mechanism to achieve spectral selectivity; common examples include gratings, prisms, and interferometers with moving mirrors. A spectroscopic technique—here dubbed Planck spectroscopy—that measures the spectral emissivity of a surface using only a temperature‐controlled stage and a detector, without any wavelength‐selective optical components is experimentally demonstrated and validated. Planck spectroscopy involves the measurement of temperature‐dependent thermally emitted power, where the spectral selectivity is realized via the temperature and wavelength dependence of Planck's law. Planck spectroscopy in the mid infrared, for wavelengths from 3 to 13 µm—limited primarily by the bandwidth of the detector—with resolution of ≈1 µm is experimentally demonstrated and validated. The minimalistic setup of Planck spectroscopy can be implemented using infrared cameras to achieve low‐cost infrared hyperspectral imaging and imaging ellipsometry.