The lack of baseline estimates of air-sea CO2 exchange in Arctic and sub-Arctic regions represents a major shortfall in our ability to understand how climate change may affect CO2 fluxes at high latitudes. The 2005 ArcticNet cruise of Hudson Bay (Canada) provided a rare comprehensive oceanographic survey of one such region. Ship-based observations of sea-surface fugacity of CO2 (fCO2sw) were made at 56 locations between 15 September and 26 October and were found to range from 259 μatm in Hudson Strait to 425 μatm at the entrance to James Bay. Strong relationships between fCO2sw and river discharge were identified, with coastal waters observed to be supersaturated with respect to the atmosphere, while offshore waters were undersaturated. High correlations of fCO2sw with salinity, sea surface temperature, and colored dissolved organic matter suggest that thermodynamic effects and the oxidation of riverine carbon are driving supersaturation in the coastal zone. Calculated instantaneous fluxes of CO2 ranged from +16.5 mmol m−2 d−1 in James Bay to −19.6 mmol m−2 d−1 in Foxe Channel. Using National Centers for Environmental Prediction wind speed climatologies, a net sink in Hudson Bay of −0.73 (±0.4) mmol m−2 d−1 was estimated for study period, substantially lower compared to many other Arctic shelf environments. This initial study provides a preliminary examination of fCO2sw dynamics in Hudson Bay; future analyses and field measurements will be necessary to properly constrain CO2 fluxes in this season and over an annual cycle.