Portsmouth is a town in Newport County, Rhode Island, United States. The population was 17,871 at the 2020 U.S. census. Portsmouth is the second-oldest municipality in Rhode Island, after Providence; it was one of the four colonies which merged to form the Colony of Rhode Island and Providence Plantations, the others being Providence, Newport, and Warwick.

Newport County is one of five counties located in the U.S. state of Rhode Island. As of the 2020 census, the population was 85,643. It is also one of the seven regions of Rhode Island. The county was created in 1703. Like all of the counties in Rhode Island, Newport County no longer has any governmental functions. All of those functions in Rhode Island are now carried out either by the state government, or by the cities and towns of Rhode Island. Newport County is included in the Providence-Warwick, RI-MA Metropolitan Statistical Area, which is in turn constitutes a portion of the greater Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area.