Abstract Supra‐permafrost groundwater (SPGW) is a significant but understudied source of dissolved organic matter (DOM) to Arctic coastal systems. We applied ramped pyrolysis/oxidation (RPO) with Δ14C and δ13C isotopic analysis to assess DOM reactivity and origin in SPGW, river, and lagoon waters along the Alaska Beaufort Sea coast. Activation energy (E) profiles revealed shared low‐E and mid‐E components (∼150, 170, 185 kJ mol−1), underscoring compositional ubiquity across sites. However, SPGW exhibited a pronounced peak at 170 kJ mol−1, indicative of biodegradable carbon, while lagoon DOM showed a high‐E peak at 204 kJ mol−1, suggesting oxidative aging and stabilization. Isotopic signatures signal SPGW (640–1,020 yBP) and lagoon DOM (745–995 yBP) were derived from older carbon pools, contrasting the modern signatures of riverine DOM. These results highlight SPGW as a dominant late‐season source of reactive DOM to lagoons, demonstrating a novel framework for tracing DOM transformation and fate in Arctic coastal environments.