The islands, a popular spot for walkers at Lakeside Boulevard, have been shut by Doncaster Council to help protect nesting wildlife in the area.
Signs warning visitors of no public access have been erected near the islands which are connected to the circular route around the edge of the lake by bridges.
The notice says: "Doncaster Metropolitan Borough Council has taken the decision to fence off access to the islands due to nesting wildlife.
"This is for the safety of both members of the public and the wildlife.
"Once the nesting season is over, the council will re-open the islands."
The closure also comes after a recent spell of hot weather in which a number of people were spotted jumping into the lake from the bridges and islands to cool off.