Portsmouth is the second-oldest municipality in Rhode Island, after Providence, which was founded in 1636. In 1637, Roger Williams, the founder of Providence Plantations, had negotiated with Canonicus and Miantonomi, sachems of the Narragansett, to allow a group of religious dissenters from Massachusetts Bay Colony to settle on Aquidneck Island.

Portsmouth was settled the following year by twenty-three men and their families who dissented from Boston in pursuit of religious freedom. This group of men had established Portsmouth with the signing of the Portsmouth Compact in Boston on March 7, 1638.

We whose names are underwritten do hereby solemnly in the presence of Jehovah incorporate ourselves into a Bodie Politick and as He shall help, will submit our persons, lives and estates unto our Lord Jesus Christ, the King of Kings and Lord of Lords and all those perfect and most absolute laws of His given us in His Holy Word of Truth, to be guided and judged thereby.

Portsmouth Compact

These men and their families settled in the northernmost part of Aquidneck Island along a brook. In 1936, a memorial park known as Founders Brook was dedicated to honor the founders of the town. A plaque commemorating the Portsmouth Compact is affixed to a large boulder known as a "Pudding Stone."

In December 1776 during the American Revolutionary War, British forces occupied Aquidneck Island.

In 1778, events in New York encouraged American and French forces to try to recapture the island and overtake the British forces in Newport.

The Battle of Rhode Island took place on the hot and humid morning of August 29, 1778. The Continental Army, led by Major General John Sullivan, was forced to retreat to the north end of the island in Portsmouth. With reinforcements, British Major General Robert Pigot pursued the retreating American forces. Sullivan managed to ferry his troops across the Sakonnet River and into Tiverton.

The British occupied the island until October 1779.

Nearly one-hundred years later, Portsmouth became home to the U.S. Army’s Lovell General Hospital from 1862 to 1865 where hundreds of sick and wounded Union and Confederate soldiers were housed and treated. Today an active marina and shipbuilding company occupies the site.

The late 19th century gave birth to a number of gentlemen’s farms throughout Portsmouth, where prize-winning cattle and horses and other domesticated animals were raised.

The second-half of the 20th century saw increased development within the town of Portsmouth. In 1990, the Aquidneck Land Trust was formed to preserve and save Aquidneck Island’s remaining open spaces.

Today, in the early 21st Century, the town is fully modernized with established neighborhoods throughout. Some old traditions have been discarded, yet, the old occupations of fishing, farming and boat-building continue, often in different ways. New traditions include that of the Newport International Polo Series held in Portsmouth along East Main Road across from a former Vanderbilt farm.