This paper relates an infrared (IR) (6000–2500; 1400–400 cm−1) spectroscopic study of sideronatrite, Na2Fe3+(SO4)2(OH)·3H2O. Spectra in the 600–1250 cm−1 range are dominated by fundamental S–O absorption bands. Factor-group analysis based on the space group P212121 () is in accordance with the observed band multiplicity. A broad and convolute absorption centred around 3400 cm−1 and a sharp band at 3605 cm−1 are observed in the 2900–3900 cm−1 frequency region. The resolution of the broad band is significantly improved at liquid nitrogen temperature; seven components can be fitted to the pattern and these can be assigned to H2O/OH molecules in the structure. All components in the broad band and the sharp absorption at 3605 cm−1 are strongly polarized for E//a, in accordance with the structure analysis results. Infrared transmission spectroscopy is a useful tool to identify these sulfate minerals occurring in specific geological environments.