The Amazon River (‘Río de las Amazonas’) has the highest discharge of any river in the world and is also the longest or second longest. Its source has therefore intrigued scholars and explorers for centuries. A river's source is often defined as the most distant upstream point in the drainage basin. For the past several decades, the longest upstream extension of the Amazon River has been held to be the Nevado Mismi area of the Río Apurímac drainage. We overturn this longstanding view by employing topographic maps, satellite imagery, digital hydrographic datasets and GPS tracking data to show that the Cordillera Rumi Cruz (10.7320°S, 76.6480°W; elevation ∼5220 m) in the Río Mantaro drainage lies 75–92 km further upstream than Nevado Mismi. We compare various methods for measuring each of the Río Apurímac and Río Mantaro lengths, and show that high-resolution satellite imagery and GPS tracking most closely follow the path of the water. Our results reposition the ‘most distant source’ of the Amazon to a more tropical location, change the uppermost ∼800 km of river to this point, and add 75–92 km to the river's maximal length.