Manresa Island is a former island located in Norwalk, Connecticut, at the mouth of Norwalk Harbor in the Long Island Sound. The earliest name for the landform was Boutons Island, which dates to 1664. By the 19th century, the island had been purchased by John H. Keyser, who lived there and cultivated rare vegetation. He built a causeway in 1860, connecting the island to the mainland of South Norwalk. This gave rise to the name Keyser Island, which was sold to the Jesuits toward the end of the century.