Lake Texcoco (Spanish: Lago de Texcoco; Nahuatl languages: Tetzco(h)co) was a natural saline lake within the Anahuac or Valley of Mexico. Lake Texcoco is best known for an island situated on the western side of the lake where the Mexica built the city of Mēxihco Tenōchtitlan, which would later become the capital of the Aztec Empire. After the Spanish conquest, efforts to control flooding led to most of the lake being drained.

The entire lake basin is now almost completely occupied by Mexico City, the capital of the present-day nation of Mexico. Drainage of the lake has led to serious ecological and human consequences. The local climate and water availability have changed considerably, contributing to water scarcity in the area; subsequent groundwater extraction leads to land subsidence under much of the city. Native species endemic to the lake region, such as the axolotl, have become severely endangered or extinct due to ecosystem change.

The Valley of Mexico is a large natural basin located in the southern highlands of Mexico's central altiplano, with an average elevation of 2,236 m (7,336 ft) above mean sea level. Lake Texcoco formerly extended over a large portion of the southern half of this basin, where it was the largest of an interconnected chain of five major and several smaller lakes (the other major lakes being Lakes Xaltocan, Zumpango, Chalco, and Xochimilco). Much of the lake was fed by groundwater aquifers; freshwater poured in from Lake Chalco and Xochimilco's freshwater springs, while the thermal springs of Zumpango and Xaltocan, as well as some in Texcoco itself, provided saline water. During periods of high water levels—typically after the May-to-October rainy seasons—the lakes were often joined as one body of water, at an average elevation of 2,242 m (7,356 ft) above mean sea level. In the drier winter months the lake system tended to separate into individual bodies of water, a flow that was mitigated by the construction of dikes and causeways in the Late Postclassic period (1200–1521 CE) of Mesoamerican chronology. Lake Texcoco was the lowest-lying of all the lakes, and occupied the minimum elevation in the valley so that water ultimately drained towards it. The Valley of Mexico is a closed or endorheic basin. Because there is no outflow, evapotranspiration is estimated to be 72–79% of precipitation.

Between the Pleistocene epoch and the last glacial period, the lake occupied the entire Mexico Valley. Lake Texcoco reached its maximum extent 11,000 years ago with a size of about 2,189 square miles (5,670 km²) and over 500 feet (150 m) deep. When the lake's water level fell it created several paleo-lakes that would connect with each other from time to time. At the north in the modern community of San Miguel Tocuilla there is an important paleontological field where many fossilized Pleistocene fauna have been discovered. The lake was primarily fed by snowmelt from nearby mountain glaciers when the Mexico Valley had a temperate climate. Between 11,000 and 6,000 years ago, the climate naturally warmed and snowfall in central Mexico became less prevalent. This caused the water level of the lake to drop over the next several millennia. Remnants of the ancient shoreline that Lake Texcoco had from the last glacial period can be seen on some slopes of Mount Tlaloc as well as mountains west of Mexico City. The disarticulated remains of seven Columbian mammoths dated between 10,220 ± 75 and 12,615 ± 95 years (BP) were found, suggesting human presence. It is believed that the lake may have disappeared and subsequently re-formed at least 10 times in the last 30,000 years.

Agriculture around the lake began about 7,000 years ago, with humans following the patterns of periodic inundations of the lake. Several villages appeared on the northeast side of the lake between 1700 and 1250 BC. By 1250 BC the identifying signs of the Tlatilco culture, including more complex settlements and a stratified social structure, are seen around the lake. By roughly 800 BC Cuicuilco had eclipsed the Tlatilco cultural centers and was the major power in the Valley of Mexico during the next 200 years when its famous conical pyramid was built. The Xitle volcano destroyed Cuicuilco around AD 30, a destruction that may have contributed to the rise of Teotihuacan.

After the fall of Teotihuacan, AD 600–800, several other city-states appeared around the lake, including Xoloc, Azcapotzalco, Tlacopan, Coyohuacan, Culhuacán, Chimalpa, and Chimalhuacán – mainly from Toltec and Chichimeca influence. None of these predominated and they coexisted more or less in peace for several centuries. This time was described as a Golden Age in Aztec chronicles. By the year 1300, however, the Tepanec from Azcapotzalco were beginning to dominate the area.

According to a traditional story, the Mexica wandered in the deserts of modern Mexico for 100 years before they came to the thick forests of the place now called the Valley of Mexico.

Tenochtitlan was founded on an islet in the western part of the lake in the year 1325. Around it, the Aztecs created a large artificial island using a system similar to the creation of chinampas. To overcome the problems of drinking water, the Aztecs built a system of dams to separate the salty waters of the lake from the rain water of the effluents. It also permitted them to control the level of the lake. The city also had an inner system of channels that helped to control the water.