Rexburg is the principal city of the Rexburg, ID Micropolitan Statistical Area, which includes Fremont and Madison Counties.
As of the census of 2000, there were 17,257 people, 4,274 households, and 2,393 families residing in the city. The population density was 3,534.4 people per square mile (1,365.4/km²). There were 4,533 housing units at an average density of 928.4/sq mi (358.6/km²). The racial makeup of the city was 95.20% White, 0.30% African American, 0.31% Native American, 0.66% Asian, 0.28% Pacific Islander, 2.23% from other races, and 1.03% from two or more races. Hispanic or Latino of any race were 4.04% of the population.
There were 4,274 households out of which 30.5% had children under the age of 18 living with them, 47.7% were married couples living together, 5.9% had a female householder with no husband present, and 44.0% were non-families. 12.7% of all households were made up of individuals and 5.5% had someone living alone who was 65 years of age or older. The average household size was 3.71 and the average family size was 3.45.
In the city the population was spread out with 18.3% under the age of 18, 57.3% from 18 to 24, 11.9% from 25 to 44, 7.5% from 45 to 64, and 4.9% who were 65 years of age or older. The median age was 20 years. For every 100 females there were 82.4 males. For every 100 females age 18 and over, there were 77.0 males.
The median income for a household in the city was $26,965, and the median income for a family was $36,047. Males had a median income of $27,280 versus $17,592 for females. The per capita income for the city was $9,173. About 13.2% of families and 44.4% of the population were below the poverty line, including 11.7% of those under age 18 and 12.3% of those age 65 or over.
Rexburg is often referred to as the "reddest place in America," owing to the area's strong conservative majority and political trends. Since 1980 no Republican presidential candidate has failed to carry the county with less than 59 percent of the vote. In that same period Republican presidential candidates polled more than 90 percent of the county's vote on two occasions, Ronald Reagan in 1984 and George W. Bush in 2004. John McCain came close to this level in 2008, drawing 85 percent of the vote.
Rexburg leads the country in the number of pet schnauzers per capita. According to the American Kennel Club, on average, there is one schnauzer for every 6.12 people in the city. The next highest schnauzer city is Orono, Maine at one schauzer per 7.38 people.