The Okanogan National Forest is a U.S. National Forest located in northern and western Okanogan County in north-central Washington State, United States. The 1,499,013-acre (2,342.2 sq mi, or 6066.3 km²) forest is bordered on the north by Canada, on the east by Colville National Forest, on the south by the divide between the Methow and the Stehekin-Lake Chelan valleys, and on the west by North Cascades National Park. The closest significant communities are Omak and Okanogan. Managed by the Forest Service together with Wenatchee National Forest, its headquarters is in Wenatchee.
There are local ranger district offices located in Tonasket and Winthrop. It is the second-largest National Forest (after the Nez Perce National Forest in Idaho) that is contained entirely within one county. Most of the Pasayten Wilderness (excluding its westernmost part, which lies in Mount Baker National Forest), and the northeast portion (about 63%) of Lake-Chelan-Sawtooth Wilderness are part of the forest, with the balance lying in Mount Baker NF.
The western part of the forest is wetter than the dry and less temperate east. The vegetation varies similarly, from the western boreal forest, to the eastern high-elevation steppe. A 1993 United States Forest Service study estimated that the extent of old growth in the Forest was 316,000 acres (128,000 ha), a majority of which was Lodgepole Pine forests. Wildfires are not uncommon in the Okanogan National Forest. Notable fires include the 2006 Tripod Complex and the 2012 Okanogan Complex.
Okanogan National Forest was established on July 1, 1911 from a portion of Chelan National Forest. On July 1, 1921 the entire forest was transferred back to Chelan, but on March 23, 1955 Chelan became Okanogan.