Abstract. Extreme warming events are increasingly more intense and frequent in the global ocean. These events are predicted to drive profound and widespread effects on marine ecosystems, yet their impact on phytoplankton, the base of the marine food web, is still largely unknown. Our understanding of the impact of these phenomena in marine ecosystems is particularly poor in the remote and logistically challenging Southern Ocean. During summer 2020, the research vessel Hespérides sampled the water column of the Drake Passage and northern Antarctic Peninsula before (early January) and during the early phase (late January-early February) of a Marine Heat Wave (MHW), that resulted in sea surface temperature anomalies of up to +3 °C. Here, we take advantage of this exceptional opportunity to document the effects of an extreme warming event on the nutrient and phytoplankton (diatom and coccolithophores) distributions across the main zonal systems of the Southern Ocean. Overall, our results indicate that biogeographical variability of diatom and coccolithophore assemblages, the two dominant phytoplankton groups in the Southern Ocean, mirrored the physical and chemical properties of the water masses delineated by the Southern Ocean fronts before and during the onset of the marine heat wave. Analysis of a suite of satellite-derived oceanographic parameters revealed that development and persistence of the 2020 marine heat wave were closely tied to mesoscale anticyclonic eddy dynamics. The increase in sea surface temperatures during the onset of the marine heat wave was associated with a remarkable increase in diatom abundance reaching bloom concentrations and a shift in the diatom assemblage towards an increase in the relative abundance of the small diatom Fragilariopsis cylindrus/nana in the southern Drake Passage. Notably, the diatom bloom was not coupled with a statistically significant change in chlorophyll a, as derived from in-situ fluorescence, or modelled Net Primary Production. It is likely that the differing contribution of other phytoplankton groups and/or a shift in the average phytoplankton size before and during the MHW might be responsible for these results. Average coccolithophore abundance was lower than previous studies in the Drake Passage and decreased during the MHW. We speculate that the remarkable nitrate decrease by approximately one order of magnitude lower than average summer concentrations might have been responsible for the reduction in coccolithophore numbers. Low nitrate levels are attributed to either the advection of nitrate poor waters from lower latitudes by an anticyclonic eddy and/or nutrient consumption by substantial development of soft-tissue phytoplankton biomass. Overall, our results reinforce the notion that a warmer Southern Ocean will favour an increase of small phytoplankton cells in the southern Drake Passage and northern Antarctic Peninsula with unpredictable consequences in the marine-food web and biogeochemical cycles that need to be urgently quantified and parametrized.