- Umatilla County, Oregon (data.census.gov)
Umatilla County, Oregon has 3,215.4 square miles of land area and is the 8th largest county in Oregon by total area. Umatilla County, Oregon is bordered by Walla Walla County, Washington, Union County, Oregon, Columbia County, Washington, Wallowa County, Oregon, Morrow County, Oregon, Grant County, Oregon, and Benton County, Washington.