Methane-consuming microbes inhabiting marine methane seeps have recently been found to have the capacity to assimilate inorganic nitrogen, suggesting a previously unaccounted role in the global nitrogen cycle. Despite ex-situ experimental observations, definitive evidence of this process under in-situ conditions remains elusive, hindering the complete understanding of the controlling factors and magnitude of this process. We present the isotopic variations of organic carbon d¹³C_org and total nitrogen d¹⁵N values in two sediment cores collected from the gas hydrate-bearing Håkon Mosby Mud Volcano, SW Barents Sea (72°N, ~1260 m water depth). We identified a stratigraphic interval containing methane-derived carbonates directly overlying a gas hydrate layer at 67 cm and typified by d¹³C_org and d¹⁵N as low as -42.0‰ and 1.2‰, respectively. Stable isotope mixing models confirm in-situ nitrogen uptake by methanotrophic consortia, contributing to up to 49.1 wt% of the local bulk sedimentary organic matter – a finding calling for reevaluation of the role of methane seeps in the oceanic nitrogen cycle.