Lake Muir is a freshwater lake, with a larger surrounding wetlands area, that is located in the South West region of Western Australia. The lake lies near Muirs Highway, north of Walpole and southeast of Manjimup. The lake has a surface area of 46 square kilometres (18 sq mi).
Lake Muir and its surrounding wetland lies within the Lake Muir-Unicup System, a 694 square kilometres (268 sq mi) area of internal drainage containing a complex of wetland systems. Lake Muir may, in flood, overflow southwest into the Deep River catchment (and possibly also southeast into the Frankland River via Poorginup Gully).
Lake Muir is usually brackish (1000–3000 mg/L TDS) at the end of winter, saline by summer and dry throughout autumn.