Micro X-ray fluorescence mapping and X-ray absorption near-edge structure spectra were successfully measured for selenium (Se) in barite formed in marine sediments, which showed that Se(IV) was mainly found in the barite. The results showed that (i) Se in natural barite can be detected by mu-XRF-XAFS, (ii) the presence of Se(IV) suggested that the barite was precipitated under suboxic conditions, and (iii) the oxidation state of Se in barite can be used as a redox indicator of the depositional environment.