Large solitary ascidians, like Herdmania grandis (Heller), can dominate the benthic substrates of subtropical and temperate reefs, however their influence on nitrogen cycling, particularly nitrous oxide (N2O) production, is unknown. Here we incubated individual H. grandis and compared fluxes of dissolved inorganic and gaseous nitrogen species to fluxes from reef sediments. Nitrous oxide production rates per individual ascidian (21 ± 8 nmol ind h-1) are the highest reported for any marine invertebrate. An individual ascidian produced more N2O than a m2 of sediment (1.7 ± 1.7 nmol m-2 h-1). Ascidian mediated N2O production was found to occur under nutrient depleted conditions. While the addition of 15N labelled organic material showed that H. grandis facilitates both nitrification and denitrification, the contribution of these pathways to N2O production could not be ascertained. As the ecology of temperate reefs change, any range expansion of H. grandis will increase coastal N2O production.