Itzcoatl (Classical Nahuatl: Itzcōhuātl [it͡sˈkoːwaːt͡ɬ], "Obsidian Serpent," modern Nahuatl pronunciation ) was the fourth king of Tenochtitlan, ruling from 1427 (or 1428) to 1440, the period when the Mexica threw off the domination of the Tepanecs and laid the foundations for the eventual Aztec Empire.
Itzcoatl was an illegitimate son of tlàtoāni Acamapichtli and an unknown Tepanec woman from Azcapotzalco. He was elected as the king when his predecessor, his nephew Chimalpopoca, was killed by Maxtla of the nearby Tepanec āltepētl (city-state) of Azcapotzalco. Allying with Nezahualcoyotl of Texcoco, Itzcoatl went on to defeat Maxtla and end the Tepanec domination of central Mexico.
After this victory, Itzcoatl, Nezahualcoyotl, and Totoquilhuaztli, king of Tlacopan, forged what would become known as the Aztec Triple Alliance, forming the basis of the eventual Aztec Empire.
Itzcoatl next turned his attention to the chinampas districts on the south shores of Lakes Xochimilco and Chalco. Fresh water springs lining these shores had allowed the development of extensive raised gardens, or chinampas, set on the shallow lake floors. Successful campaigns against Xochimilco (1430), Mixquic (1432), Cuitlahuac (1433), and Tezompa would secure agricultural resources for Tenochtitlan and, along with the conquest of Culhuacan and Coyoacán, would cement the Triple Alliance's control over the southern half of the Valley of Mexico.
With this string of victories, Itzcoatl took the title Culhua teuctli, "Lord of the Culhua" while Totoquilhuaztli, king of Tlacopan, took the title Tepaneca teuctli, "Lord of the Tepanecs".
In 1439, Itzcoatl undertook a campaign outside the Valley of Mexico against Cuauhnahuac (Cuernavaca).
According to the Florentine Codex, Itzcoatl ordered the burning of all historical codices because it was "not wise that all the people should know the paintings". Among other purposes, this allowed the Aztec state to develop a state-sanctioned history and mythos that venerated Huitzilopochtli.
Itzcoatl also continued the building of Tenochtitlan: during his reign temples, roads, and a causeway were built. Itzcoatl established the religious and governmental hierarchy that was assumed by his nephew Moctezuma I upon his death in 1440.
Itzcoatl was a son of Acamapichtli and half-brother of Huitzilihuitl. He was an uncle of Chimalpopoca and Moctezuma I.
He married princess Huacaltzintli and had a son Tezozomoc