Achromobacter denitrificans PR1 has previously shown potential to degrade the antibiotic sulfamethoxazole, whereby sulfamethoxazole biotransformation was stimulated in the presence of biogenic substrates. This study examined the biotransformation kinetics of sulfamethoxazole and its two main conjugates, N4-acetyl-SMX and SMX-N1-Glucuronide, by activated sludge and activated sludge bioaugmented with A. denitrificans PR1. SMX biotransformation under both anoxic and aerobic conditions was tested, with and without the addition of acetate as growth substrate, to understand the range of applicable conditions for bioaugmentation purposes. Biological process models, such as the pseudo-first order kinetic and cometabolic models, were also applied and, following the estimation of kinetic parameters, could well describe data measured in bioaugmented and non-bioaugmented AS batch experiments under various test conditions. Experimental and modelling results suggest that (i) retransformation of the two conjugates to SMX in AS occurred under both aerobic and anoxic conditions, and (ii) biotransformation kinetics of SMX can vary significantly depending on redox conditions, e.g., SMX was biotransformed by AS only under aerobic conditions. Notably, SMX biotransformation was significantly enhanced when PR1 was bioaugmented in AS. Addition of acetate as biogenic substrate is not necessary, as PR1 was capable of enhancing the SMX biotransformation by using the carbon sources present in wastewater. Overall, bioaugmentation by means of A. denitrificans PR1 could be a viable strategy for enhancing SMX removal in AS wastewater treatment plants (WWTPs).