Water stress in the Amazon is exacerbated by rising temperatures and reduced moisture levels. However, understanding forest responses to increased aridity is hindered by limited in-situ water potential observations in the Amazon. Remote sensing of water content has emerged as a promising metric. Vegetation Water Content (VWC) diurnal dynamics is hypothesized to reflect water stress responses. Conventional sensors’ low sampling rates impede capturing and studying sub-daily VWC dynamics. Leveraging Global Navigation Satellite System Reflectometry (GNSS-R) with unprecedented sampling rates, this study reveals significant disparities in morning and evening VWCs in the Amazon, e.g., by ≈ 1.1 and 1.0 kg/m2 during the wet and dry seasons of 2019. A strong correlation (R = 0.8) between ΔVWC (the difference between evening and morning VWCs) and vapor pressure deficit is observed in Amazonian peatland. This highlights the potential of VWC from innovative remote sensing techniques in elucidating water stress dynamics in critical ecosystems.