St. Petersburg is a city in Pinellas County, Florida, United States. It is known as a vacation destination for both American and foreign tourists. As of the 2010 census, the population was 244,769, making St. Petersburg the fourth most populous city in the state of Florida and the largest city in Florida that is not a county seat. Although the city of Clearwater is the county seat of Pinellas County, all county services are available through county offices in St. Petersburg. St.