Phytoplankton size structure, a major determinant of trophic structure and biogeochemical functioning in pelagic ecosystems, can be described by the slope of the size-abundance spectrum (SAS). Previous observational studies reporting spatio-temporal changes in phytoplankton SAS slope have focused on particular open-ocean or coastal environments. Therefore, the overall variability in phytoplankton SAS slope still has not been investigated over wide ranges of biomass and productivity including both oligotrophic open-ocean regions and productive coastal waters. Here we present a multi-cruise overview of the basin-scale variability in phytoplankton biomass and SAS slope across the Atlantic Ocean, covering coastal, shelf, and oceanic environments over the 50°N-50°S latitude range. We find the inverse relationship between cell size and abundance to be pervasive across the studied regions, even in highly productive coastal waters. In oceanic regions, consistent latitudinal patterns are observed in the relationship between nutricline depth, phytoplankton biomass and SAS slope. There is a strong degree of covariation between SAS slope at the surface and at the base of the euphotic layer, indicating that geographical changes in phytoplankton size structure override vertical variability. A basin-scale relationship exists between increasing resource supply, enhanced phytoplankton biomass, and progressively less steep SAS slopes, reflecting increasing importance of large cells in more productive waters. However, the relationship between ecosystem productivity and both SAS slope and mean community cell size is saturating, which means there is no continuous trend towards ever increasing contribution by larger cells. Similar phytoplankton size structures, with a biomass dominance by the 2–20 μm size class, are found in both moderately and highly eutrophic waters. Our results provide an observational benchmark for testing the predictions of size-based plankton models and for assessing future, climate-related shifts in phytoplankton size structure in both coastal and oceanic regions of the Atlantic Ocean.