Tyrosine sulfation, a well-characterized post-translation modification in eukaryotes, has not previously been reported in prokaryotes. Here, we demonstrate that the RaxST protein from the Gram-negative bacterium, Xanthomonas oryzae pv. oryzae, is a tyrosine sulfotransferase. We used a newly developed sulfotransferase assay and ultraviolet photodissociation mass spectrometry to demonstrate that RaxST catalyses sulfation of tyrosine 22 of the Xoo Ax21 (activator of XA21-mediated immunity) protein. These results demonstrate a previously undescribed post-translational modification in a prokaryotic species with implications for studies of host immune responses and bacterial cell–cell communication systems.