Despite the significance of sulfur in controlling arsenic cycling, thioarsenate formation has not been studied in paddy soils, yet. Here, we combined lab incubation experiments with field porewater sampling from two Italian paddy soils to assess the geochemical significance of thioarsenate. Incubation data showed formation of both inorganic and methylated thioarsenates under flooded conditions, even in the absence of extra sulfate addition. Percentages of total arsenic were 12.5% and 18.3%, respectively. All species were also discovered after draining the systems for 4 days and re-flooding. Field sampling at rice flowering stage further confirmed the existence of inorganic (average 22.3%) and methylated thioarsenates (average 2.2%) under natural conditions. In summary, since our results conclusively prove the occurrence and quantitative importance of thioarsenate in paddy soils, their environmental fate urgently needs to be addressed.