Amazon forests have experienced multiple large-scale droughts in recent decades, which have increased tree mortality and reduced carbon sequestration. However, the extent to which drought sensitivity varies across Amazonian forests and its key controls remain poorly quantified. Here, we analyse satellite remotely-sensed Solar Induced Fluorescence anomalies to investigate responses in Amazon forest photosynthetic activity to the 2015-2016 El Nino-Southern Oscillation (ENSO) drought. Using multivariate regression analysis, we examine the relative importance of ENSO-associated climate anomalies, background climate and soil characteristics in controlling basin-wide forest photosynthetic activity differences. Our model explains 25% of forest photosynthetic response and indicates background climate and soil conditions had a greater influence than the climatic anomalies experienced. We find marked sensitivity differences across Amazonia, with North-Western forests being the most sensitive to precipitation anomalies, likely relating to variation in forest species composition and background water stress. Such factors should be considered in climate change impact simulations.