Waterloo is a census-designated place in San Joaquin County, California.

[2] The 2020 United States census reported Waterloo's population was 670.

According to the United States Census Bureau, the CDP covers an area of 5.4 square miles (14.1 km2), all of it land.

38 households (18.1%) were made up of individuals, and 21 (10.0%) had someone living alone who was 65 years of age or older.

There were 224 housing units at an average density of 41.2 per square mile (15.9/km2), of which 160 (76.2%) were owner-occupied, and 50 (23.8%) were occupied by renters.