Hydroxylation of atrazine to nontoxic hydroxyatrazine is generally considered an efficient detoxification method to remediate atrazine-contaminated soil and water. However, previous studies suggested that hydroxylation was not the dominant pathway for atrazine degradation in the hydroxyl radical-generating systems such as Fenton reaction, ozonation and UV/H2O2. Herein we report that the addition of sodium thiosulfate can realize rapid hydroxylation of atrazine to hydroxyatrazine at pH ≤ 4 under room temperature. High resolution mass spectra and isotope experiments results revealed that the hydroxylation of atrazine was involved with nucleophilic substitution and subsequent hydrolysis reaction as follows. HS2O3-, as a species of thiosulfate only at pH ≤ 4, first attacked C atom connecting to chlorine of atrazine to dechlorinate atrazine and produce C8H14N5S2O3-. Subsequently, the S-S bond of C8H14N5S2O3- was cleaved easily to form SO3 and C8H14N5S-. Next, C8H14N5S- was hydrolyzed to generate hydroxyatrazine and H2S. Finally, the comproportionation of SO3 and H2S in situ produced S0 during hydroxylation of atrazine with thiosulfate. This study clarifies the importance of degradation pathway on the removal of pollutants, and also provides a nonoxidative strategy for atrazine detoxification in seconds.