Abstract Warming can lead to mobilization of organic matter (OM) initially stored in circumarctic permafrost and subsequent greenhouse gas release to the atmosphere. Our understanding remains limited regarding how the extent of carbon release, that is, OM reactivity, varies across terrestrial permafrost types and how it changes during transport from land to marine shelves. In this study, we measured bulk organic (TOC, C/N), isotopic ( δ 13 C, Δ 14 C), and thermogravimetric properties (TGA) as proxies of OM reactivity on bulk and water‐soluble fractions (leachates) from terrestrial Holocene and Pleistocene permafrost, bulk surface sediments from the Laptev Sea, and sediment cores from the western Laptev Sea. Bulk OM from terrestrial Pleistocene permafrost exhibited lower reactivity compared to Holocene permafrost, as indicated by its lower thermoreactivity and more advanced degradation state, reflected in higher δ 13 C values and lower C/N ratios. Marine surface sediments showed relatively old radiocarbon ages and reduced OM thermoreactivity in the eastern Laptev Sea shelf compared to the central and western Laptev Sea shelf. This likely resulted from a higher contribution of Pleistocene permafrost‐derived OM. In the central and western Laptev Sea, a rapid decrease in OM thermoreactivity was observed near the coast, followed by a more gradual decline offshore. Downcore analyses revealed that the reduction in OM thermoreactivity primarily reflected degradation during cross‐shelf transport rather than after burial. Our results advance the understanding of OM reactivity differences between Pleistocene and Holocene permafrost, as well as changes in terrestrial permafrost OM thermoreactivity during transport and post‐burial. Plain Language Summary Warming in the Arctic is releasing carbon from permafrost as microbes break down frozen soil, plants, and animal remains. To better understand this process, we studied how easily organic matter (OM) from both young and old terrestrial permafrost breaks down, as well as OM in surface and deeper sediments from the Laptev Sea shelf. We measured how easily the OM in these materials breaks down by examining the amount of the OM that decomposed at lower versus higher temperatures. On land, we found that old permafrost is more resistant to breakdown than young permafrost. In the eastern Laptev Sea shelf, surface sediments contained more OM from old, less degradable terrestrial permafrost. Moving westward, OM degradability decreases with increasing distance from the coast, suggesting rapid OM breakdown nearshore. Deeper sediment layers revealed that most OM degradation happened during transport from land to sea and across the shelf, with relatively little breakdown after burial. These findings improve our understanding of the differing vulnerabilities of young and old permafrost to degradation and emphasize the importance of transport processes in shaping OM reactivity distributions on Arctic Ocean marginal shelves. Key Points Bulk organic matter and leachates from terrestrial Pleistocene permafrost exhibit lower thermoreactivity than Holocene permafrost Organic matter source and transport distance on shelf collectively control organic matter thermoreactivity in shelf surface sediment The majority of reactive organic matter from terrestrial permafrost is degraded during transport on land or in nearshore coastal areas