Who was the first European to navigate the Amazon river from the Andes to the Atlantic Ocean?

The first European to navigate the Amazon from almost the waterhead to the mouth was the Spaniard Francisco de Orellana, an alleged cousin of the infamous conquistador Francisco Pizarro.

In 1540, another Pizarro was charged with an expedition to locate the "El Dorado", a city allegedly overflowing with gold and riches, somewhere to the east. Orellana was one of Gonzalo Pizarro's lieutenants. Within a month of setting off, much of the group had perished in the harsh and inhospitable tropical rainforest east of the Andes.

Orellana was ordered to follow a river and find food. He became stranded and had not found supplies, so he continued with a small number of men down the Rio Napo. The further he went downriver, the more inhospitable the jungle became. Despite the problems, he followed the Napo until its confluence with the Amazon and kept drifting with the current until the group emerged in the Atlantic in August 1542.

The Portuguese explorer Pedro Teixeira was the first European to complete the return voyage of the Amazon from the mouth to Quito and back, he started in 1637 and returned to the Atlantic by1639.

Vicente Yáñez Pinzón was the first acknowledged European to sail up the Amazon River. He made it some 50 miles from the sea.

Charles Marie de La Condamine, French geographer and mathematician, led the first scientific exploration of the Amazon. In 1743 he published the first map of the Amazon based on actual astronomical observations.