Lake Ilsanjo is a man-made lake located in Annadel State Park east of Santa Rosa in Sonoma County, California, United States.

Entrepreneur Joe Coney originally owned the land that is now Annadel State Park and built the lake in the mid-1950s, naming it after his wife Ilsa and himself.

Originally the overflow drained to a gully during the winter months, but in 2003, the state spent almost US$400,000 building a spillway and release valve to reduce downstream erosion.

During the winter of 2007–08, park officials accidentally left the release valve open, causing the lake level to drop far below normal, exposing mudflats that would normally be underwater.

This geographical article related to Sonoma County, California is a stub.