The Amazon might also be the world's longest river—depending on whom you ask. Most scientists believe the South American river is at least 4,000 miles (6,400 km) long—still shorter than the Nile, which is widely held to be the world's longest river at about 4,132 miles (6,650 km).

The debate over whether the Nile or the Amazon is the world's longest river continues to be a subject of scientific contention in 2026. Historically, the Nile has been credited as the longest at approximately 4,132 miles (6,650 km). However, recent studies using satellite imagery and revised mapping of the Amazon's source in the Peruvian Andes suggest that the Amazon might actually be longer, measuring roughly 4,345 miles (6,992 km). The discrepancy arises from the difficulty in defining a river's "true" source and mouth, particularly for the Amazon, which has a complex delta system and multiple seasonal headwaters. While the Nile remains the traditional answer in many textbooks, many geographers now argue that the Amazon is not only the largest river by volume and basin size but also the longest in terms of continuous water flow. Until a single international geographic body officially settles on a standard measurement methodology for river sources, both rivers can technically claim the title depending on which scientific study or encyclopedia (such as Britannica versus various Brazilian research institutes) you choose to cite.