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is a town in Newport County, Rhode Island, United States. The population was 17,149 at the 2000 census.
According to the United States Census Bureau, the town has a total area of 59.3 square miles (153.6 km²), of which, 23.2 square miles (60.1 km²) of it is land and 36.1 square miles (93.5 km²) of it (60.86%) is water. Most of its land area lies on Aquidneck Island, which it shares with Middletown and Newport. In addition, Portsmouth encompasses some smaller islands, including Prudence Island, Patience Island, Hope Island, and Hog Island.