Abstract Hydrothermal systems at ultraslow‐spreading ridges are poorly understood, especially where thick sediments overlay detachment faults. The International Ocean Discovery Program Expedition 403 obtained the longest sediment record in Atlantic‐Arctic ultraslow‐spreading systems, reaching ∼628 m below the seafloor. Heat‐flow measurements, fluid geochemistry, and mineralogical analyses, combined with seismic interpretation, revealed thermohydraulic systems within two contourite drifts on ridge flanks. The Svyatogor Ridge site exhibits extreme heat flow and crustal‐fluid migration from two detachment faults, accompanied by newly documented thermogenic hydrocarbons, resulting in stratified thermohydraulic systems. The Vestnesa Ridge site exhibits moderate heat flow, and partial crustal‐fluid migration suggests developing thermohydraulic systems. The results reveal complex, long‐lived hydrothermal systems in the present day with significant, previously poorly understood reservoirs of geological carbon and suggest their million‐year‐scale evolution. This study advances understanding of fluid dynamics, fluid‐sediment interactions, and carbon cycling in sedimented ultraslow‐spreading ridge environments.