Umatilla County is a county located in the U.S. state of Oregon. It is included in the 8 county definition of Eastern Oregon. The county is named for the Umatilla River. In 2010, its population was 75,889. The county seat is Pendleton.
Umatilla County is part of the Pendleton–Hermiston Micropolitan Statistical Area.
Umatilla County was created on September 27, 1862, out of a portion of Wasco County. Adjustments were made to the county's boundaries following the creation of Grant, Morrow, Union, and Wallowa Counties. This legislative act also designated Marshall Station as the temporary county seat. An 1865 election selected Umatilla City, now known as Umatilla, as the county seat. With the development of wheat farming, population shifted to the north and east parts of the county, and a subsequent election in 1868 moved the county seat again to Pendleton.
The Umatilla Indian Reservation was established by the Treaty of Walla Walla in 1855. The Umatillas, Walla Wallas, and Cayuse tribes were resettled there, and is located immediately southeast of Pendleton.
EZ Wireless of Hermiston officially opened on February 4, 2004, one of the largest known Wi-Fi wide area networks in the United States, covering parts of Umatilla County, Morrow County and Benton County, Washington. Although created to facilitate communications among local police, firemen and EMT workers who immediately respond to possible accidents or terrorist attacks on the Umatilla Chemical Depot, where the U.S. Army maintained a national arsenal of nerve gas, the network can be accessed in some places by the public for free.