The Southwestern Atlantic Ocean (SAO), is considered as one of the most productive areas of the world, with high abundance of ecologically and economical important fish species. Yet, the biological responses of this complex region to climate variability are still uncertain. Here, using 24 years of satellite derived Chl-a datasets, we classified the SAO into coherent regions based on homogeneous temporal variability of Chl-a concentration, as revealed by the SOM (Self-Organizing Maps) analysis. These coherent biogeographical regions were the basis of our regional trend analysis in phytoplankton biomass, regional phenological indices, and environmental forcing variations. A generalized positive trend in phytoplankton concentration is observed, especially in the highly productive areas of the northern shelf-break, where phytoplankton biomass is increasing at an outstanding rate up to 0.42 ± 0.04 mg m-3 per decade associated with the sea surface temperature (SST) warming (0.11 ± 0.02 °C decade-1) and the mixed layer depth shoaling (-3.36 ± 0.13 m decade-1). In addition to the generalized increase in chlorophyll, the most sticking changes in phytoplankton dynamics observed in the SAO are related to the secondary bloom that occurs in most of the regions (15 ± 3 and 24 ± 6 days decade-1) which might be explained by the significant warming trend of SST, which would sustain the water stratification for a longer period, thus delaying the secondary bloom initialization. Consistent with previous studies, our results provided further evidences of the impact of climate change in these highly productive waters.