As of the census of 2000, there were 12,182 people, 3,921 households, and 3,164 families residing in the county. Zapata County is estimated to be the eleventh fastest growing county (+15.8%) in the state of Texas since the year 2000 (based on % of population change). The population density was 12 people per square mile (5/km²). There were 6,167 housing units at an average density of 6 per square mile (2/km²). The racial makeup of the county was 84.07% White, 0.41% Black or African American, 0.32% Native American, 0.19% Asian, 0.04% Pacific Islander, 12.64% from other races, and 2.33% from two or more races. 84.78% of the population were Hispanic or Latino of any race.
There were 3,921 households out of which 43.20% had children under the age of 18 living with them, 64.20% were married couples living together, 13.00% had a female householder with no husband present, and 19.30% were non-families. 17.50% of all households were made up of individuals and 10.30% had someone living alone who was 65 years of age or older. The average household size was 3.10 and the average family size was 3.52.
In the county, the population was spread out with 33.00% under the age of 18, 10.00% from 18 to 24, 24.10% from 25 to 44, 18.60% from 45 to 64, and 14.30% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 96.80 males. For every 100 females age 18 and over, there were 93.70 males.
The median income for a household in the county was $24,635, and the median income for a family was $26,722. Males had a median income of $26,294 versus $14,579 for females. The per capita income for the county was $10,486. About 29.30% of families and 35.80% of the population were below the poverty line, including 46.10% of those under age 18 and 21.30% of those age 65 or over.