AbstractExtant climate observations suggest the dry season over large parts of the Amazon Basin has become longer and drier over recent decades. However, such possible intensification of the Amazon dry season and its underlying causes are still a matter of debate. Here we used oxygen isotope ratios in tree rings (δ¹⁸O_TR) from six floodplain trees from the western Amazon to assess changes in past climate. Our analysis shows that δ¹⁸O_TR of these trees is negatively related to inter-annual variability of precipitation during the dry season over large parts of the Amazon Basin, consistent with a Rayleigh rainout model. Furthermore δ¹⁸O_TR increases by approximately 2‰ over the last four decades (~ 1970–2014) providing evidence of an Amazon drying trend independent from satellite and in situ rainfall observations. Using a Rayleigh rainout framework, we estimate basin-wide dry season rainfall to have decreased by up to 30%. The δ¹⁸O_TR record further suggests such drying trend may not be unprecedented over the past 80 years. Analysis of δ¹⁸O_TR with sea surface temperatures indicates a strong role of a warming Tropical North Atlantic Ocean in driving this long-term increase in δ¹⁸O_TR and decrease in dry season rainfall.