The effects of deltamethrin, in the presence and absence of nitrate, on soil microbial activity (as reflected by the rates of soil microbial basal respiration, denitrification, and methanogenesis) were studied in a riparian wetland soil under both aerobic and anaerobic conditions. A microcosm study was carried out with soil collected from the vicinity of a wetland. The soil was then amended with 50, 125, and 250 mg deltamethrin kg−1 dry weight soil, in the presence and absence of 20 mg N-NO3− kg−1 dry weight soil. Half-life values for deltamethrin degradation ranged from 27 to 291 days, depending on experimental conditions. Nitrates had an inhibitory effect on deltamethrin degradation. Deltamethrin, under anaerobiosis, had an inhibitory effect on soil respiration; this effect was reversed in the presence of nitrate. An antagonistic effect between deltamethrin degradation and denitrification activity was observed. In the presence of nitrate, the activation of denitrifying bacteria led to competitive inhibition of methanogens. It was concluded that deltamethrin, designed to affect specific functions of its target organisms, also has an effect on nontarget organisms, that is, the soil microbial community. To our knowledge, this is the first report on the degradation and environmental impact of deltamethrin in a riparian wetland soil.