Rattlesnake Lake is a lake in the northwest United States, located in Rattlesnake Mountain Scenic Area in King County, Washington, approximately thirty miles (50 km) east of Seattle, south of Interstate 90.

The watershed supplies 65% of the Seattle region’s unfiltered drinking water to nearly 800,000 people.

However, Rattlesnake Lake itself is not used for drinking water and is spring-fed by the nearby Cedar River.

The Rattlesnake Ledge Hiking Trail ascends 1,160 feet (350 m) over two miles (3 km)[3] of well maintained switchbacks from the north shore of the lake to the scenic Rattlesnake Ledge viewpoint that overlooks the lake.

The spacious, grassy shores around the lake are used for many outdoor activities, including slacklining and picnicking.