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 and fastest growing 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.