Seffner is a census-designated place (CDP) in Hillsborough County, Florida, United States. The population was 7,579 at the 2010 census.
According to the Seffner Chamber of Commerce, the exact date of Seffner's founding is unknown. Some historians claim that Seffner's streets were originally laid out in 1862, but not recorded as a plat formally until 1885. Seffner's post office was opened in 1884, once a new railroad line in the area had been planned. The community was in fact named for its first postmaster, F.P. Seffner. That same year, Seffner's first school and store were opened. The main thoroughfare leading north and south was Lenna Avenue and leading east and west was Highway Number 23, renamed Buffalo Avenue, because of its erstwhile use as a buffalo trail, and then renamed again in 1989 to its current name, Dr. Martin Luther King, Jr. Boulevard (Hwy 574). Seffner (and neighboring Mango and Dover) served as the three primary towns along the South Florida Railroad right-of-way between Tampa and Plant City. These towns owe their existence and prosperity to the railroad, each becoming an important shipping center.