Seffner is a census-designated place (CDP) in Hillsborough County, Florida, United States. The population was 7,579 at the 2010 census, although its ZIP codes (33584 & 33583), whose boundaries extend beyond the ones given by the census and which include the places of Mango and a portion of Thonotosassa, have a population of around 35,000.Approximately 15 miles east of downtown Tampa, Seffner serves as a suburban haven for commuters.