Lake Illawarra is a large coastal lagoon, near the city of Wollongong about 100 km south of Sydney, New South Wales.

The towns of Dapto and Shellharbour are near the lake, which receives the runoff from the Illawarra escarpment through the Macquarie Rivulet.

Lake Illawarra has a narrow tidal entrance to the sea at Windang. The lake is shallow and vulnerable to pollution and urban run-off. It was used as a source of cooling water for the now-closed Tallawarra Power Station.