Diamondhead Lake is an unincorporated community and census-designated place in Penn Township, Guthrie County, Iowa, United States. As of the 2020 census the population was 371.

The CDP consists of a residential community that surrounds Diamondhead Lake, an artificial impoundment just south of the South Raccoon River. The community is located in the southeast corner of Guthrie County, 4 miles (6 km) northwest of Dexter and 6 miles (10 km) northeast of Stuart. It is 38 miles (61 km) west of Des Moines.

According to the U.S. Census Bureau, the CDP has a total area of 0.77 square miles (2.0 km), of which 0.62 square miles (1.6 km) are land and 0.15 square miles (0.4 km), or 20.31%, are the waters of Diamondhead Lake.

Historical population

Census	Pop.	Note	%±
2010	366		—
2020	371		1.4%
U.S. Decennial Census

As of the census of 2020, there were 371 people, 154 households, and 117 families residing in the community. The population density was 381.3 inhabitants per square mile (147.2/km). There were 226 housing units at an average density of 232.3 per square mile (89.7/km). The racial makeup of the community was 90.6% White, 0.8% Black or African American, 0.8% Native American, 1.9% Asian, 0.5% Pacific Islander, 0.3% from other races and 5.1% from two or more races. Hispanic or Latino persons of any race comprised 2.4% of the population.

Of the 154 households, 27.3% of which had children under the age of 18 living with them, 69.5% were married couples living together, 3.2% were cohabitating couples, 12.3% had a female householder with no spouse or partner present and 14.9% had a male householder with no spouse or partner present. 24.0% of all households were non-families. 21.4% of all households were made up of individuals, 9.7% had someone living alone who was 65 years old or older.

The median age in the community was 48.9 years. 25.6% of the residents were under the age of 20; 3.0% were between the ages of 20 and 24; 18.1% were from 25 and 44; 31.0% were from 45 and 64; and 22.4% were 65 years of age or older. The gender makeup of the community was 47.7% male and 52.3% female.