Rivers play an important role in the transport of carbon (C) into the global ocean, and en route much C is lost to the atmosphere, buried, or transformed largely by biological or photochemical processes. Many Arctic and subarctic watersheds drain seasonally, or permanently frozen soils, rich in organic C. In particular, the rivers of southwestern Hudson Bay and James Bay (SHBJB) drain the Hudson Bay Lowlands (HBL), which are the world’s second largest contiguous wetland and are estimated to hold in excess of 33 Gt C. The amount of terrestrial C delivered from the HBL into the adjacent coastal ocean, its seasonality and composition (i.e., organic/inorganic) has yet to be quantified. This knowledge gap has prevented a comprehensive understanding of food web dynamics and nutrient cycling in estuarine and marine environments of SHBJB. Since 2021, and through community partnerships, the river C load has been monitored in the lower reaches of the Moose River watershed, and thereafter C monitoring has been expanded to include other major rivers draining the HBL. Here we present information on the delivery of terrestrial C from major rivers into SHBJB resulting from the monitoring program, and provide, for the first time, estimates of total riverine C delivery from the HBL into James Bay. Our goal is to better understand the response of the land-to-ocean C delivery in the HBL to changes in watersheds attributed to climate and development, and to support ecosystem studies within SHBJB.