Oxygen availability impacts the marine nitrogen cycle at a range of spatial and temporal scales. Invasive organisms have shown to sustainably affect sediment geochemistry and benthic ecology. Nonionella sp. T1 was recently described as an invasive benthic foraminifer in the North Sea region. Here, we demonstrate the impact of this denitrifying species on the foraminifera fauna and the nitrogen cycle of the Gullmar Fjord (Sweden). The foraminifera contribution to benthic denitrification was estimated by coupling living foraminifera micro-distribution, denitrification rate measurement and sedimentary nitrate 2D distribution. Nonionella sp. T1 dominated the foraminifera fauna and could denitrify up to 50–100 % of nitrate porewater in oxygenated bottom waters of the fjord. Contrastingly, at the deepest hypoxic low-nitrate station, denitrifying foraminifera species were scarce and did not contribute to nitrogen removal (~ 5 %). Our study showed that benthic foraminifera can be a major contributor of nitrogen mitigation in oxic coastal ecosystems and should be included in ecological and diagenetic models aiming at understanding biogeochemical cycles coupled to nitrogen.