As of the 2020 census, the population was 3,704,[1] making it Iowa's least-populous county.

Its original size was later reduced by the creation of Montgomery and Union counties.

[1] As of the 2010 United States census, there were 4,029 people, 1,715 households, and 1,126 families residing in the county.

[11] In terms of ancestry, 33.6% were German, 15.9% were Irish, 14.7% were English, and 4.9% were American.

[14] The population ranking of the following table is based on the 2020 census of Adams County.