In late 2023, the Amazon River Basin experienced its most extreme drought, putting its population and ecosystem at major risks. Gauges that were still functioning measured the lowest river water levels (RWL) on record. Here, satellite observations, including Surface Water Ocean Topography (SWOT), reveal the spread and timing of extremely low RWL across the entire river system. Nadir altimeter observations show that the 2023 minimum RWL in Central Amazon were more than 3 m below their annual average, representing two to three times its mean variability. Additionally, SWOT enables the first basin-scale characterization of the reduction in RWL with a spatial resolution of 200 m and how it propagates with time. Large-scale evaluation with gauges suggest that SWOT outperforms classical altimetry in estimating RWL, even if observed biases need further investigations. SWOT offers a new opportunity to understand hydroclimatic extremes and their broad impacts on the environment of the Amazon.