Lake Poinsett is a census-designated place in Hamlin and Brookings counties in South Dakota, United States. The population was 501 at the 2020 census. The Lake Poinsett CDP comprises the lake of the same name and its shoreside communities in Hamlin and Brookings counties. U.S. Route 81 passes through the CDP along the western shore of th…Lake Poinsett is a census-designated place in Hamlin and Brookings counties in South Dakota, United States. The population was 501 at the 2020 census. The Lake Poinsett CDP comprises the lake of the same name and its shoreside communities in Hamlin and Brookings counties. U.S. Route 81 passes through the CDP along the western shore of the lake and leads north 24 miles to Watertown and south 13 miles to Arlington.