Jacksonville is the most populous city proper in the U.S. state of Florida, located on the Atlantic coast of northeastern Florida. It is the seat of …Jacksonville is the most populous city proper in the U.S. state of Florida, located on the Atlantic coast of northeastern Florida. It is the seat of Duval County, with which the City of Jacksonville consolidated in 1968. It was the largest city by area in the contiguous United States as of 2020, and became the 10th largest city by population in 2023.