Portsmouth is a town in Newport County, Rhode Island, United States. The population was 17,389 at the 2010 U.S. Census. According to the United States Census Bureau, the town has a total area of 59.3 square miles (154 Square Kilometer), of which, 23.2 square miles (60 Square Kilometer) of it is land and 36.1 square miles (93 Square Kilometer) 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.