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Fenpyrazamine (FPA) is a widely used fungicide in agriculture to control fungal diseases, but its environmental degradation by oxidants and the formation of potential degradation products remain unexplored. This study investigates the oxidation of FPA by hydroxyl radicals (HO˙) using density functional theory (DFT) calculations at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G(d,p) level of theory. Three standard oxidation mechanisms, including formal hydrogen transfer (FHT), radical adduct formation (RAF), and single electron transfer (SET), were evaluated in the aqueous phase, with reaction kinetics analyzed over a temperature range of 283–333 K. As a result, the reactivity order of the mechanisms was determined to be RAF > FHT > SET. At 298 K, the calculated total rate constants for FHT and RAF reactions were competitive, being 6.09 × 109 and 8.21 × 109 M−1 s−1, respectively, while that for SET was slightly lower at 2.35 × 109 M−1 s−1. The overall rate constant was estimated to be 1.67 × 1010 M−1 s−1. The most favourable RAF reaction occurred at the C38[double bond, length as m-dash]C39 double bond, while the predominant FHT reactions involved the H15 and H13 hydrogen atoms of the methyl C8 group. The lifetime of FPA in natural water with respect to HO˙ oxidation was predicted to range from 10.84 hours to 2.62 years, depending on environmental conditions. Furthermore, the toxicity assessments revealed that while FPA is neither bioaccumulative nor mutagenic, it poses developmental toxicity and is harmful to aquatic organisms, including fish, daphnia, and green algae.