South County is a local euphemism for Washington County, Rhode Island's southern-most county. In virtually all local parlance, "South County" is the preferred nomenclature.
This usage is so widespread that, despite the fact that, officially, there is no such thing as South County, businesses, organizations and other institutions are so-named. Examples include South County Hospital and the South County Bike Path, both of which are sanctioned by the state. There are even highway signs directing motorists to "South County Beaches."
Because counties have few official purposes in Rhode Island, this naming convention matters almost not at all. For outsiders and newcomers, it can be confusing.