Arctic warming is expected to increase the terrestrial dissolved organic carbon flux into the central Arctic Ocean, altering biogeochemical cycling by modulating light attenuation, microbial respiration and carbon dioxide release. Quantifying terrestrial inputs remains challenging due to biases in common proxies and uncertainties in endmember characteristics, which complicate traditional mixing models, introducing uncertainties in predicting climate change impacts. Here we present a high-resolution mass spectrometric approach allowing direct analysis of original seawater, tracing and quantifying terrestrial contributions to dissolved organic carbon. Terrestrial dissolved organic carbon in the central Arctic Ocean contributed at least 0.97 ± 0.05 PgC (16.4%) to the dissolved organic carbon inventory of 5.93 ± 0.09 PgC, including 15.0% in deep water (7.9 ± 0.4 µmol l−1). In surface water within the Transpolar Drift, the average terrestrial dissolved organic carbon concentrations were 117% higher (31.5 ± 4.8 µmol l−1) than outside the Transpolar Drift (14.5 ± 1.0 µmol l−1). The terrestrial dissolved organic matter is compositionally distinct, being more aromatic, hydrophobic and nitrogen-poor than marine sources. This approach provides chemical information that reflects changes in organic matter sources and bioavailability, both of which are central to understanding future climatic impacts on Arctic biogeochemical cycles.