Umatilla County is one of the 36 counties in the U.S. state of Oregon. As of the 2020 census, the population was 80,075. 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 92,261. It is included in the eight-county definition of Eastern Oregon.