The Roosevelt–Rondon Scientific Expedition (Portuguese: Expedição Científica Rondon-Roosevelt) was jointly led by Theodore Roosevelt and Cândido Rondon in 1913–14 to be the first Old World explorers of the 1000-mile long "River of Doubt" (later renamed Rio Roosevelt) located in a remote area of the Brazilian Amazon basin which had previously been explored by the indigenous people of the area. Sponsored in part by the American Museum of Natural History, they also collected many new animal and insect specimens.

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Roosevelt had originally planned to go on a speaking trip of Argentina and Brazil, followed by a cruise of the Amazon River. Instead, the Brazilian Government suggested that Roosevelt accompany famous Brazilian explorer Cândido Rondon on his exploration of the previously unknown River of Doubt, the headwaters of which had only recently been discovered. Roosevelt, seeking adventure and challenge after his recent defeat for a third term in the White House, agreed. Kermit Roosevelt, Theodore's son, had recently become engaged and did not plan on joining the expedition but did on the insistence of his mother, in order to protect his father. The expedition started in Cáceres, a small town on the Paraguay River, with 15 Brazilian porters (camaradas), the two leaders, Roosevelt's son, and American naturalist George Cherrie. They traveled to Tapirapuã, where Rondon had previously discovered the Headwaters of the River of Doubt. From Tapirapuã, the expedition traveled northwest, through dense forests and then later through the plains on top of the Parecis plateau. They reached the River of Doubt on February 27, 1914.

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Almost from the start, the expedition was fraught with problems. Insects and disease such as malaria weighed heavily on just about every member of the expedition, leaving them in a constant state of sickness, festering wounds and high fevers. The heavy dug-out canoes were unsuitable to the constant rapids and were often lost, requiring days to build new ones. The food provisions were ill-conceived forcing the team on starvation diets. Natives (the Cinta Larga) shadowed the expedition and were a constant source of concern—the Indians could have at any time wiped out the expedition and taken their valuable metal tools but they chose to let them pass (future expeditions in the 1920s were not so lucky).

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Of the 19 men who went on the expedition, only 16 returned. One died by accidental drowning in rapids with his body never being recovered, one died by murder and was buried at the scene, and the murderer was left behind in the jungle, presumably swiftly perishing there.

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By the time the expedition had made it only about one-quarter of the way down the river, they were physically exhausted and sick from starvation, disease and the constant labour of hauling canoes around rapids. Roosevelt himself was near death as a wounded leg had become infected and the party feared for his life each day. 

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After Roosevelt returned, there was some doubt that he had actually discovered the river and made the expedition. Even though he was still quite weak and barely able to speak above a whisper, Roosevelt, angry that his credibility had been challenged, arranged speaking engagements with the National Geographic Society in Washington, D.C. on May 26, and the Royal Geographical Society in London in mid-June. These appearances largely stifled the criticisms at the time. To finally settle the dispute, in 1927 American explorer George Miller Dyott led a second trip down the river, confirming Roosevelt's discoveries.

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