Lake Hillier is a saline lake on the edge of Middle Island, the largest of the islands and islets that make up the Recherche Archipelago off the south coast of Western Australia. It is particularly notable for its pink colour. A long and thin shore divides the Southern Ocean from the lake.

Lake Hillier is about 600 metres (2,000 ft) in length by about 250 metres (820 ft) in width. The lake is surrounded by a rim of sand and a dense woodland of paperbark and eucalyptus trees with a narrow strip of sand dunes covered by vegetation separating its northern edge from the northern coast of Middle Island. The most notable feature of the lake is its pink colour. The vibrant colour is permanent, and does not alter when the water is taken in a container. The source of the pink colour is considered to be due to the presence of the organism Dunaliella salina. Air is the best mode of transportation for viewing the lake.

In the north-eastern part was a small lake of a rose colour, the water of which, as I was informed by Mr. Thistle who visited it, was so saturated with salt that sufficient quantities were crystallised near the shores to load a ship. The specimen he brought on board was of a good quality, and required no other process than drying to be fit for use. Flinders visited Middle Island again in May 1803; he intended “to stop a day or two in Goose-Island Bay, for the purposes of procuring geese for our sick people, seal oil for our lamps, and a few casks of salt from the lake on Middle Island”. It is reported that Flinders subsequently named the lake after William Hillier, a crew member of Investigator who died of dysentery on 20 May 1803 prior to the expedition's departure from Middle Island.