Waterloo is a city in and the county seat of Black Hawk County, Iowa, United States. It is the eighth-most populous city in Iowa, with a population of 67,314 at the 2020 census. Waterloo comprises a twin conurbation with neighboring municipality Cedar Falls, and is the larger of the two cities. The Waterloo – Cedar Falls metropolitan area has an estimated 170,000 residents.

Waterloo is a major manufacturing, commercial, and cultural center in Northeast Iowa. Locally known as the "Factory City," John Deere brand agricultural machinery has been produced in the city for over a century. Waterloo is bisected by the Cedar River flowing southeastward.

Waterloo was originally known as Prairie Rapids Crossing. The town was established near two Meskwaki American tribal seasonal camps alongside the Cedar River. It was first settled in 1845 when George and Mary Melrose Hanna and their children arrived on the east bank of the Red Cedar River (now just called the Cedar River). They were followed by the Virden and Mullan families in 1846. Evidence of these earliest families can still be found in the street names Hanna Boulevard, Mullan Avenue and Virden Creek.

On December 8, 1845, the Iowa State Register and Waterloo Herald was the first newspaper published in Waterloo.

The name Waterloo supplanted the original name, Prairie Rapids Crossing, shortly after Charles Mullan petitioned for a post office in the town. Since the signed petition did not include the name of the proposed post office location, Mullan was charged with selecting the name when he submitted the petition. Tradition has it that as he flipped through a list of other post offices in the United States, he came upon the name Waterloo. The name struck his fancy, and a post office was established under that name. There were two extended periods of rapid growth over the next 115 years. From 1895 to 1915, the population increased from 8,490 to 33,097, a 290% increase. From 1925 to 1960, population increased from 36,771 to 71,755. The 1895 to 1915 period was a time of rapid growth in manufacturing, rail transportation and wholesale operations. During this period the Waterloo Gasoline Traction Engine Company moved to Waterloo and, shortly after, the Rath Packing Company moved from Dubuque. Another major employer throughout the first two-thirds of the 20th century was the Illinois Central Railroad. Among the others was the less-successful brass era automobile manufacturer, the Maytag-Mason Motor Company.

On June 7, 1934, bank robber Tommy Carroll had a shootout with the FBI when he and his wife stopped to pick up gas. Accidentally parking next to a police car and wasting time dropping his gun and picking it back up, Carroll was forced to flee into an alley, where he was shot. He was taken to Allen Memorial Hospital in Waterloo, where he soon died.

Waterloo suffered in the agricultural recession of the 1980s; its major employers at the time were heavily rooted in agriculture. John Deere, the area's largest employer, cut 10,000 jobs, and the Rath meatpacking plant closed altogether, losing 2,500 jobs. It is estimated that Waterloo lost 14% of its population during this time. Today the city enjoys a broader industrial base, as city leaders have sought to diversify its industrial and commercial mix. John Deere remains a large employer for Waterloo, but employs only roughly one-third the number of people it did at its peak. Layoffs in 2024 and 2025 further reduced John Deere's presence in the city.

In 1910, black railroad workers were brought in as strikebreakers to the Waterloo area. Black workers were relegated to 20 square blocks in Waterloo, an area that remains the east side to this day. In 1940, more black strikebreakers were brought in to work in the Rath meat plant. In 1948, a black strikebreaker killed a white union member. Instead of a race riot, a strike ensued against the Rath Company. The National Guard was called in to end the 73-day strike.