The island was named Wells Island but renamed by Captain William Fitzwilliam Owen to the current name. There is no indication whom it was named for, but likely for Richard Wellesley, 1st Marquess Wellesley.
The island is located St. Lawrence River, which surrounds Wellesley Island on three sides. The island bounds an internal body of water, the Lake of the Isles, which nearly doubles its waterfront. The easternmost peninsula of the island lies across the Upper (American) Narrows from the Village of Alexandria Bay.in the
An arterial highway, Interstate 81, crosses Wellesley Island. Five segments composing the Thousand Islands International Bridge link the island to the opposite shores of the river. Said to be "the smallest vehicular international bridge in the world," one of the bridges crosses a narrow channel, the International Rift, which separates Wellesley Island in the United States from Hill Island in Canada. The United States border-crossing facility at the port of entry is located on the northeastern part of the island.
Wellesley, one of the largest of the Thousand Islands in the St. Lawrence River, is home to a growing number of full-time residents. Population increases dramatically during the summer months. The island has two State Parks, a nature center, and three golf courses. Located on its southern tip is the Thousand Island Park Historic District, listed on the National Register of Historic Places in 1982. In addition to Interstate 81, which connects two portions of the Thousand Islands Bridge, Jefferson County Routes 100 and 191 serve the island.
- Barnett Marsh—A swamp in the south part of the island.
- Densmore Bay—A bay on the south shore of the southeast peninsula.
- Eel Bay—A large bay between Wellesley Island and Grindstone Island.
- Lake of the Isles—A body of water connected to the St. Lawrence River, lying between the northeast and southeast peninsulas.
- South Bay—A bay on the southwest part of the island.
Communities on the island
- Moore Landing
- Thousand Island Park
- Westminster Park
Parks on the island
- Wellesley Island State Park—In several locations on the island.
- Dewolf Point State Park—Along Lake of the Isles on the southeast side of the northern peninsula.
- Waterson Point State Park—On the northern edge of the island near the Canada–US border.
- Mary Island State Park—Not actually on Wellesley Island but accessible only by water, this park is separated by a small channel from the easternmost tip of Wellesley Island
- National Park Service (2009-03-13). "National Register Information System". National Register of Historic Places. National Park Service.