According to the United States Census Bureau, the city has a total area of 1.6 square miles (4.0 km²), all of it land.
Buellton is on the Santa Ynez River in the Santa Ynez Valley, and in 2004 was one of the fastest-growing towns in central Santa Barbara County. It is a common stop for travelers on U.S. Highway 101, being the first town north of Santa Barbara after the scenic and undeveloped stretch of about 25 miles (40 km) through the Gaviota coast.
As of the census of 2000, there were 3,828 people, 1,433 households, and 1,000 families residing in the city. The population density was 2,450.4 people per square mile (947.4/km²). There were 1,483 housing units at an average density of 949.3/sq mi (367.0/km²). The racial makeup of the city was 81.50% White, 0.55% African American, 1.15% Native American, 1.10% Asian, 0.21% Pacific Islander, 12.23% from other races, and 3.27% from two or more races. Hispanic or Latino of any race were 25.73% of the population.
There were 1,433 households out of which 34.7% had children under the age of 18 living with them, 58.2% were married couples living together, 8.0% had a female householder with no husband present, and 30.2% were non-families. 23.9% of all households were made up of individuals and 10.7% had someone living alone who was 65 years of age or older. The average household size was 2.67 and the average family size was 3.17.
In the city the population was spread out with 27.1% under the age of 18, 6.7% from 18 to 24, 29.6% from 25 to 44, 23.0% from 45 to 64, and 13.6% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 98.5 males. For every 100 females age 18 and over, there were 94.7 males.
The median income for a household in the city was $48,490, and the median income for a family was $54,839. Males had a median income of $46,379 versus $28,542 for females. The per capita income for the city was $20,907. About 6.6% of families and 8.8% of the population were below the poverty line, including 10.2% of those under age 18 and 7.3% of those age 65 or over.