The Forrestdale and Thomsons Lakes Ramsar Site comprises two separate nature reserves, totalling 754 ha (1,860 acres) in area, protecting two shallow fresh to brackish, seasonal lakes in a suburban and agricultural landscape in south-western Western Australia.

It lies in the Swan Coastal Plain bioregion and is used mainly for birdwatching and walking.

They are similar in size and shape, being oval, about 1.6 km (0.99 mi) long by 1.3 km (0.81 mi) wide, with large central areas of open water when full and with shorelines vegetated with concentric fringes of the introduced bulrush Typha orientalis, sedges, paperbarks and other plants tolerant of seasonal waterlogging.

They are the best remaining examples of brackish, seasonal lakes with extensive fringing sedgeland typical of the Swan Coastal Plain.

Regionally they form a major breeding, migration stop-over and semi-permanent drought refuge area for shorebirds and other waterbirds.