Waco (/ˈweɪkoʊ/ WAY-koh) is a city in and the county seat of McLennan County, Texas, United States. It is situated along the Brazos River and I-35, halfway between Dallas and Austin. The city had a U.S. census estimated 2024 population of 146,608, making it the 24th-most populous city in the state. The Waco metropolitan statistical area consists of McLennan, Falls and Bosque counties, which had a 2020 population of 295,782. Bosque County was added to the Waco MSA in 2023. The 2024 U.S. census population estimate for the Waco metropolitan area was 307,123 residents.

Indigenous peoples occupied areas along the river for thousands of years. In historic times, the area of present-day Waco was occupied by the Wichita Indian tribe known as the "Waco" (Spanish: Hueco or Huaco).

In 1824, Thomas M. Duke was sent to explore the area after violence erupted between the Waco people and the European settlers. His report to Stephen F. Austin, described the Waco village:

This town is situated on the West Bank of the river. They have a spring almost as cold as ice itself. All we want is some Brandy and Sugar to have Ice Toddy. They have about 400 acres (1.6 km²) planted in corn, beans, pumpkins, and melons and that tended in good order. I think they cannot raise more than One Hundred Warriors.

— Thomas M. Duke, Stephen F. Austin Papers

After further violence, Austin halted an attempt to destroy their village in retaliation. In 1825, he made a treaty with them. The Waco were eventually pushed out of the region, settling north near present-day Fort Worth. In 1872, they were moved onto a reservation in Oklahoma with other Wichita tribes. In 1902, the Waco received allotments of land and became official US citizens. Neil McLennan settled in an area near the South Bosque River in 1838. Jacob De Cordova bought McLennan's property and hired a former Texas Ranger and surveyor named George B. Erath to inspect the area. In 1849, Erath designed the first block of the city. Property owners wanted to name the city Lamartine, but Erath convinced them to name the area Waco Village, after the Indians who had lived there. In March 1849, Shapley Prince Ross, the father of future Governor Lawrence Sullivan Ross, built the first house in Waco, a double-log cabin, on a bluff overlooking the springs. His daughter Kate was the first settler child born in Waco. Because of this, Ross is considered to have been the founder of Waco, Texas.

In 1866, Waco's leading citizens embarked on an ambitious project to build the first bridge to span the wide Brazos River. They formed the Waco Bridge Company to build the 475-foot (145 m) brick Waco Suspension Bridge, which was completed in 1870. The company commissioned a firm owned by John Augustus Roebling in Trenton, New Jersey, to supply the bridge's cables and steelwork and contracted with Mr. Thomas M. Griffith, a civil engineer based in New York, for the supervisory engineering work. The economic effects of the Waco bridge were immediate and large. The cowboys and cattle-herds following the Chisholm Trail north, crossed the Brazos River at Waco. Some chose to pay the Suspension Bridge toll, while others floated their herds down the river. The population of Waco grew rapidly, as immigrants now had a safe crossing for their horse-drawn carriages and wagons. Since 1971, the bridge has been open only to pedestrian traffic and is in the National Register of Historic Places.

Waco was the original intended western terminus of the Texas and St. Louis Railway, with the town having been reached in 1881. However, the line was extended further west to Gatesville a year later. This trackage later became the core of the St. Louis Southwestern Railway Company, commonly known as the Cotton Belt.