There are 9 US Post Offices in Martin County serving a total of 186,154 residents equating to 20,684 residents per post office. It's estimated that approximately 297,123 packages are handled across the 9 county post offices annually. Martin County is a county located in the Treasure Coast region in the state of Florida, in the United States. As of the 2010 census, the population was 146,318. Its county seat is Stuart. Martin County is included in the Port St. Lucie, FL Metropolitan Statistical Area, which is also included in the Miami-Fort Lauderdale-Port St. Lucie, FL Combined Statistical Area."