亚马逊河（葡萄牙语：Rio Amazonas；西班牙语：Río Amazonas），位于南美洲，是世界流量、流域最大的、支流最多的、及第二长的河流。亚马逊河流量达每秒219,000立方米，流量比其他三条大河尼罗河、长江、密西西比河的总和还要大几倍，大约相当于7条长江的流量，占世界河流流量的20%；流域面积达6,915,000平方公里，占南美洲面积的40%；支流数超过1万5千条。

由于河流长度测量方法并无定论，亚马逊河河长便有着多种不同的说法，其中之一即为常见的6,400公里，依此则稍短于非洲的尼罗河而位居世界第二长河。2007年，一群来自巴西与秘鲁的学者共同宣称找到新的源头，据此河所得的最新长度达6,992.15公里，因而超越尼罗河成为世界第一长河。但所谓「新河源」及因此而推演之「新河长」之正确性仍有待全球地理学界的检验。

The Amazon River (US /ˈæməzɒn/ or UK /ˈæməzən/; Spanish and Portuguese:Amazonas) in South America is the largest river by discharge of water in the world, and the second in length.

The headwaters of the Apurímac River on Nevado Mismi had been considered for nearly a century as the Amazon’s most distant source, until a 2014 study found it to be the Cordillera Rumi Cruz at the headwaters of the Mantaro River in Peru. The Mantaro and Apurímac confluence, and with other tributaries form the Ucayali, which in turn confluences with the River Marañón upstream of Iquitos, Peru, to form what countries other than Brazil consider to be the main stem of the Amazon. For Brazil this section of the river is the Solimões until it confluences with the Rio Negro at the Meeting of Waters (Portuguese:Encontro das Águas) at Manaus, the river's largest city.