Nitrous oxide (N2O) is an ozone-depleting gas that is 300 times more potent than carbon dioxide at heating the atmosphere. Despite continued increases in concentration, we know little about N2O dynamics in the coastal ocean. Here, we document the four-fold increase over two decades of a filter-feeding invertebrate that produces more N2O than any other marine animal examined to date in warming reefs that were previously dominated by carbon-capturing kelp. Temperature incubation experiments show that N2O production by this invertebrate increases significantly with warming. Our results indicate that one square metre of invertebrate-occupied reef produces 15% more N2O than estuarine sediment, which is noteworthy as estuaries account for ~2% of the global N2O flux. Our research shows that we are severely underestimating N2O production rates of coastal systems.