The degradation and mineralization of the plant growth regulator 2-(1-naphthyl)acetamide (NAD) was studied by excitation in the presence of the catalyst polyoxometalate decatungstate anion (W10O324−) in aqueous solution under UV (365 nm) or simulated solar light exposure. Our results indicate that the photocatalytic degradation of NAD is dependent on molecular oxygen concentration: in aerated conditions, 95% degradation was achieved after 22 h irradiation, and followed first-order kinetics with a rate constant of 3.2 × 10−3 min−1, while under de-aerated conditions almost no degradation was observed (6.0% after 22 h). Upon UV irradiation, the catalyst W10O324− enhanced NAD photodegradation by a factor of about 20 compared to its direct degradation. Oxygen appeared to play a key role on the regeneration of the catalyst, promoting the photocatalytic cycle. The primary photoproducts of NAD photocatalytic degradation were assessed by LC-ESI–MS/MS, from which a mechanism of degradation involving electron transfer and hydrogen atom abstraction is proposed. Under these conditions, mono- and di-hydroxylated and oxidized products similar to those obtained under direct photolysis have been identified. In addition, tri-hydroxylation and hydroxyl-naphthoquinone products have been identified exclusively when photolysis was carried out in presence of this catalyst. For prolonged photolysis times, it is expected that the irradiation of the tri-hydroxylated products leads to the opening of the aromatic ring and to mineralization. Furthermore, mineralization was achieved, and led to the formation of the inorganic ions NO2− (<6.0 μg L−1), NO3− (2.6 mg L−1) and NH4+ (<0.5 μg L−1).