Cuauhtémoc (Nahuatl pronunciation: [kʷaːʍˈtemoːk] ^ⓘ, Spanish pronunciation: [kwawˈtemok] ^ⓘ), also known as Cuauhtemotzín, Guatimozín, or Guatémoc, was the Aztec ruler (tlatoani) of Tenochtitlan from 1520 to 1521, and the last Aztec Emperor. The name Cuauhtemōc means "one who has descended like an eagle", and is commonly rendered in English as "Descending Eagle", as in the moment when an eagle folds its wings and plummets down to strike its prey; the name thus implies aggressiveness and determination.

Cuauhtémoc took power in 1520 as successor of Cuitláhuac and was a cousin of the late emperor Moctezuma II. His young wife, who was later known as Isabel Moctezuma, was one of Moctezuma's daughters. He ascended to the throne when he was around 25 years old, while Tenochtitlan was being besieged by the Spanish and devastated by an epidemic of smallpox brought to the Americas by Spanish conquerors. After the killings in the Great Temple, there were probably few Aztec captains available to take the position.

Cuauhtemoc's date of birth is unknown, as he does not enter the historical record until he became emperor. He was the eldest legitimate son of Emperor Ahuitzotl and may well have attended the last New Fire ceremony, marking the beginning of a new 52-year cycle in the Aztec calendar. According to several sources his mother, Tiyacapantzin, was a Tlatelolcan princess. Like the rest of Cuauhtemoc's early biography, that is inferred from knowledge of his age, and the likely events and life path of someone of his rank. Following education in the calmecac, the school for elite boys, and then his military service, he was named ruler of Tlatelolco, with the title cuauhtlatoani ("eagle ruler") in 1515. To have reached this position of rulership, Cuauhtemoc had to be a male of high birth and a warrior who had captured enemies for sacrifice. Cuauhtemoc married the Aztec princess who later became known as Isabel Moctezuma.

When Cuauhtemoc was elected tlatoani in 1520, Tenochtitlan had already been rocked by the invasion of the Spanish and their indigenous allies, the death of Moctezuma II, and the death of Moctezuma's brother Cuitlahuac, who succeeded him as ruler, but died of smallpox shortly afterwards. In keeping with traditional practice, the most able candidate among the high noblemen was chosen by vote of the highest noblemen, and Cuauhtemoc assumed the rulership. Although under Cuitlahuac Tenochtitlan began mounting a defense against the invaders, it was increasingly isolated militarily and largely faced the crisis alone, as the numbers of Spanish allies increased with the desertion of many polities previously under its control.

Cuauhtémoc called for reinforcements from the countryside to aid the defense of Tenochtitlán, after eighty days of warfare against the Spanish. Of all the Nahuas, only Tlatelolcas remained loyal, and the surviving Tenochcas looked for refuge in Tlatelolco, where even women took part in the battle. Cuauhtémoc was captured on August 13, 1521, while fleeing Tenochtitlán by crossing Lake Texcoco with his wife, family, and friends.

He surrendered to Hernán Cortés along with the surviving pipiltin (nobles) and, according to Spanish sources, he asked Cortés to take his knife and "strike me dead immediately". According to the same Spanish accounts, Cortés refused the offer and treated his foe magnanimously. "You have defended your capital like a brave warrior," he declared. "A Spaniard knows how to respect valor, even in an enemy."

At Cuauhtémoc's request, Cortés also allowed the defeated Mexica to depart the city unmolested. Subsequently, however, when the booty found did not measure up to the Spaniards' expectations, Cuauhtémoc was subjected to "torture by fire", whereby the soles of his bare feet were slowly broiled over red-hot coals, in an unsuccessful attempt to discover its whereabouts. On the statue to Cuauhtemoc, on the Paseo de la Reforma in Mexico City, there is a bas relief showing the Spaniards' torture of the emperor. Eventually, some gold was recovered but far less than Cortés and his men expected.

Cuauhtémoc, now baptized as Fernando Cuauhtémotzín, continued to hold his position under the Spanish, keeping the title of tlatoani, but he was no longer the sovereign ruler. From his surrender until his death, Cuauhtémoc was mostly kept in guarded custody by the Spaniards.