Olallie State Park, Natural state park in North Bend, Washington, US

Olallie State Park is a public recreation area in King County, Washington, stretching 3.5 miles along the South Fork of the Snoqualmie River. Its best-known feature is Twin Falls, a 135-foot waterfall. The park holds old-growth forest and five notable waterfalls, including Middle Twin Falls, Upper Twin Falls, Weeks Falls and Upper Weeks Falls.

The land was acquired by Washington State Parks in 1950 and was first known as Twin Falls State Park, a name taken from one of its most visible natural features. The park later received its current name and grew as additional land was added over the years.

The name Olallie comes from Chinook Jargon, the trade language of the region, and means berries, which grow commonly in the park. The park was renamed Olallie in 1976 after expansion, replacing its original name Twin Falls State Park.

The park is open throughout the year, but paths can become slippery in winter and sturdy footwear is a good idea at any time of year. Trails and climbing areas are most accessible from late spring through early fall, when conditions tend to be dry and manageable.

Twin Falls hides a run-of-the-river hydroelectric project that produces 24 MW of electricity, with its powerhouse 325 feet (99 m) below ground. The falls drop 135 feet (41 m), and the park holds five notable waterfalls in all: Twin Falls, Middle Twin Falls, Upper Twin Falls, Weeks Falls and Upper Weeks Falls.