Umatilla County is one of the 36 counties in the U.S. state of Oregon. The population of 81,826 ranks it as the 14th largest in Oregon, and largest in Eastern Oregon. Hermiston is the largest city in Umatilla County, but Pendleton remains the county seat. Umatilla County is part of the Hermiston-Pendleton, OR Micropolitan Statistical Area, which has a combined population of 94,833. It is included in the eight-county definition of Eastern Oregon.

Eastern Oregon is a region of the U.S. state of Oregon. It is not an officially recognized geographic entity; thus, the boundaries of the region vary according to context. It is sometimes understood to include only the eight easternmost counties in the state; in other contexts, it includes the entire area east of the Cascade Range. Cities in the basic eight-county definition include Baker City, Burns, Hermiston, Pendleton, Boardman, John Day, La Grande, and Ontario. Umatilla County is home to the largest population base in Eastern Oregon, accounting for 42% of the region's residents — more than twice that of the region’s second most populous, Malheur County. Hermiston, located in Umatilla County, is the largest city in the region, accounting for 10% of the population. Major industries include transportation/warehousing, timber, agriculture and tourism. The main transportation corridors are I-84, U.S. Route 395, U.S. Route 97, U.S. Route 26, U.S. Route 30, and U.S. Route 20.