As of the censusGR2 of 2000, there were 23,113 people, 7,247 households, and 5,667 families residing in the city. The population density was 1,652.6/km² (4,277.4/mi²). There were 8,114 housing units at an average density of 580.2/km² (1,501.6/mi²). The racial makeup of the city was 57.34% White, 4.99% Black or African American, 1.27% Native American, 5.43% Asian, 0.36% Pacific Islander, 24.48% from other races, and 6.14% from two or more races. 41.51% of the population were Hispanic or Latino of any race.
There were 7,247 households out of which 45.9% had children under the age of 18 living with them, 55.9% were married couples living together, 16.3% had a female householder with no husband present, and 21.8% were non-families. 17.6% of all households were made up of individuals and 6.9% had someone living alone who was 65 years of age or older. The average household size was 3.15 and the average family size was 3.55.
In the city the population was spread out with 34.9% under the age of 18, 10.0% from 18 to 24, 28.6% from 25 to 44, 17.4% from 45 to 64, and 9.1% who were 65 years of age or older. The median age was 28 years. For every 100 females there were 94.6 males. For every 100 females age 18 and over, there were 90.4 males.
The median income for a household in Atwater is $37,344, and the median income for a family was $39,789. Males had a median income of $32,983 versus $22,450 for females. The per capita income for the city was $15,162. About 15.3% of families and 18.7% of the population were below the poverty line, including 25.1% of those under age 18 and 5.3% of those age 65 or over.