Zebra mussels might enhance denitrification rates by altering 3 primary controls: O₂, NO₃^–, and labile C availability. We stocked mesocosms with stream sediments and either no zebra mussels (−ZM) or 10,000 mussels/m² ( ZM) to examine these potential mechanisms. We measured sediment nitrification (nitrapyrin-inhibition technique), denitrification (chloramphenicol-amended acetylene block technique), sediment O₂ profiles (microelectrodes), and a suite of water and sediment characteristics weekly for 3 wk. Nitrification (2-way analysis of variance [ANOVA], p < 0.001) and denitrification (2-way ANOVA, p < 0.001) rates were significantly higher in ZM than in −ZM mesocosms. High-NH₄ waste from zebra mussels increased sediment nitrification rates, which increased NO₃^– availability for denitrification. Furthermore, coupled nitrification–denitrification was enhanced by a reduction in the sediment depth to anoxia in the presence of zebra mussels, and this reduction allowed both processes to occur in close spatial proximity. Despite increased denitrification rates, we did not measure an increased proportion of N lost via denitrification from the ZM mesocosms. The presence of zebra mussels can increase NO₃^– availability through increased nitrification rates, potentially exacerbating eutrophication of invaded waters.