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 in the Goldfields-Esperance region, off the south coast of Western Australia. It is particularly notable for its pink colour. A long and thin shore divides the Southern Ocean (by some definitions, the Indian Ocean) from the lake.

Lake Hillier is about 600 metres (2,000 ft) in length by about 250 m (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, vibrant colour. The vibrant colour is permanent, and does not alter when the water is taken in a container. The pink colour is believed to be due to the presence of the organism Dunaliella salina. The Extreme Microbiome Project, part of the Association of Biomolecular Resource Facilities (ABRF), Metagenomics Research Group (MGRG), has performed a metagenomic analysis on the lake to find Dunaliella as well as Salinibacter ruber, Dechloromonas aromatica, and a few species of the Archaea.

Lake Hillier was visited by the Matthew Flinders expedition on 15 January 1802. Flinders's journal entries are considered to be the first written records of the lake. Flinders observed the pink lake after ascending the island's highest peak (now called Flinders Peak), describing the lake as follows: