The environmental factors influencing the microalgal bloom during sea-ice breakup in Hudson Bay were investigated during June 2018, producing the first results ever on the seasonal development of the marine ecosystem in the offshore waters of this vast inland sea. As is typical in the Arctic, primary production was found to commence at the onset of ice melt, with surface nutrient depletion leading to the formation of a subsurface chlorophyll maximum in the open waters of western Hudson Bay. Simultaneously, the melting mobile ice cover in central Hudson Bay created favorable conditions for a diatom-dominated under-ice bloom, with the results of irradiance-photosynthesis curves confirming that phytoplankton cells were acclimated to increasing light levels in the surface water. The high production rates measured in ice-covered and ice-free waters highlight the considerable plasticity of phytoplankton in terms of photosynthetic performance in this highly variable environment. Interestingly, the maximum values of primary production and phytoplankton biomass observed under the sea ice (343 mg C m-2 d-1 and 35.10 mg TChl a m-2) were lower than those observed in open waters during the late-bloom stage in the western region (486 mg C m-2 d-1 and 57.12 mg TChl a m-2), which is attributed to a confined euphotic zone (reduced light availability? Since the euphotic zone in clear waters under the ice can be as thick as elsewhere, but simply receive less irradiance overall) under the ice and low surface concentrations of inorganic nitrogen (<2 mmol L-1) in central Hudson Bay. However, the highly abundant sub-ice diatom Melosira arctica contributed an estimated additional 287 mg C m-2 d-1 to under-ice production in this region, which implies that this filamentous diatom has a similar role in the seasonally ice-covered sub-Arctic as in the central Arctic Ocean where it significantly contributes to local production. Refining the historical total production estimates of Hudson Bay with our spring observations, we recalculated annual production to be ca. 69 g C m-2, which equates to mean value for interior Arctic shelves.