Rapid Arctic warming is driving the expansion of coastal macroalgal forests, yet the role of their organic carbon in Arctic biogeochemistry remain largely unexplored. Here, isotopic, bulk, biochemical, and biomolecular analyses of surface particulate organic matter reveal that labile macroalgal carbon is a major contributor and is laterally transported with selective degradation. Continued expansion of macroalgal forests will enhance their role in Arctic carbon cycling and ecosystem functioning.