Callahan is a town in Nassau County, Florida, United States, adjacent to Duval County. The population was 1,123 at the 2010 census. It is one location of the Northeast Florida Fair.
The Battle of Alligator Bridge took place around Callahan on June 30, 1778, and was the only major engagement in an unsuccessful campaign to conquer British East Florida during the American Revolutionary War.
According to the United States Census Bureau, the town has a total area of 1.3 square miles (3.4 km2), all land.
At the 2000 census there were 962 people, 411 households, and 256 families living in the town. The population density was 724.9 inhabitants per square mile (279.3/km²). There were 444 housing units at an average density of 334.6 per square mile (128.9/km²). The racial makeup of the town was 85.86% White, 10.60% African American, 0.83% Native American, 0.21% Asian, 0.52% from other races, and 1.98% from two or more races. Hispanic or Latino of any race were 2.49%.
Of the 411 households 33.6% had children under the age of 18 living with them, 37.0% were married couples living together, 20.7% had a female householder with no husband present, and 37.5% were non-families.