Organic matter stored in Arctic permafrost represents a key component of the carbon cycle, yet its reactivity across heterogeneous continent-scale permafrost regions remains poorly understood. Here, we leverage the four shelf seas of the Eurasian Arctic as integrative receptor systems to evaluate terrestrial organic matter reactivity, assessed by examining organic carbon preservation as a function of ¹⁴C-constrained cross-shelf transport time. Our findings reveal higher reactivity of terrestrial organic matter released to the Laptev Sea and the eastern East Siberian Sea, lower reactivity in the western East Siberian Sea, and no deducible degradation in the Kara Sea. The reactivity of terrestrial organic matter is primarily determined by the degradation status and composition of its source, alongside potential microbiological controls during transport. This study reveals the heterogeneity of terrestrial organic matter reactivity across the Eurasian Arctic margin and highlights the need for detailed assessments of region-specific carbon release and modeling parameterization.