The islands are called South Bass Island, Middle Bass Island, North Bass Island, and (the much smaller) Gibraltar Island. They are located in Ottawa County in the state of Ohio.

South Bass Island is approximately 3.7 miles (6.0 km) long and 1.5 miles (2.4 km) wide and comprises 1,382 acres. South Bass Island is approximately 3.0 miles (4.8 km) north of the nearest point on the mainland (Scott Point on Catawba Island) and approximately 10 air miles (16 km) NNE of Port Clinton, Ohio and 14 air miles (23 km) NNW of Sandusky, Ohio.

The village of Put-In-Bay is located on South Bass Island. It is a popular tourist stop during the summer. Perry's Victory and International Peace Memorial, including Perry's Monument, commemorating the Battle of Lake Erie is located on South Bass Island, near Put-In-Bay.

At their closest points, Middle Bass Island is approximately 0.5 miles (0.8 km) north of South Bass Island and North Bass Island is approximately 1.0 miles (1.6 km) north of Middle Bass Island. The border between Ohio and Ontario is approximately 1.0 (1.6 km) miles north of North Bass Island.