Waterloo is a city in Linn County, Oregon, United States. The population was 229 at the 2010 census.

Linn County is one of the 36 counties in the U.S. state of Oregon. As of the 2020 census, the population was 128,610. The county seat is Albany. The county is named in the honor of Lewis F. Linn, a U.S. Senator from Missouri who advocated the American settlement of the Oregon Country. Linn County comprises the Albany, OR Metropolitan Statistical Area, which is included in the Portland-Vancouver-Salem, OR-WA Combined Statistical Area. It is located in the Willamette Valley. In 2010, the center of population of Oregon was located in Linn County, near the city of Lyons.