The Pastaza River is a large tributary to the Marañón River in the northwestern Amazon Basin of South America.

The Marañón River is the principal or mainstem source of the Amazon River, arising about 160 km to the northeast of Lima, Peru, and flowing northwest across plateaus 3,650 m high, it runs through a deeply eroded Andean valley, along the eastern base of the Cordillera of the Andes, as far as 5° 36′ southern latitude; from where it makes a great bend to the northeast, and cuts through the jungle Ande in its midcourse, until at the Pongo de Manseriche it flows into the flat Amazon basin. Although historically, the term "Marañón River" often was applied to the river all the way to the Atlantic Ocean, nowadays the Marañón River is generally thought to end at the confluence with the Ucayali River, after which most cartographers label the ensuing waterway the Amazon River.