Plum Island Light was established to guide vessels through hazardous waters at the eastern approach to Long Island Sound, especially Plum Gut. The first light station was created after the island’s west end was purchased in 1826, and a 35-foot stone tower was erected the next year. Improvements included a sixth-order Fresnel lens in 1856, but deterioration led to the present granite lighthouse, completed in 1869. Later changes reflected both evolving navigation technology and the island’s military use after Fort Terry was established in 1897. Automation ended the staffed light in 1978, yet the historic tower remains Plum Island’s best-known landmark and a lasting symbol of efforts to protect coastal navigation.