Gilliam County is a county located in the U.S. state of Oregon. The county was established in 1885 and is named for Cornelius Gilliam, who commanded the forces of the provisional government of Oregon after the Whitman Massacre. In 2010, the population was 1,871. The seat of the county is Condon.
For untold generations, American Indians traversed the county on well-worn trails to reach fishing, hunting, foraging, and trading areas; but had claimed no tribal lands. Many of these trails are still visible in the rangeland. The first non-native people in the area were Americans following the Oregon Trail to the Willamette Valley. In the late 19th century, settlers arrived from the midwestern and eastern United States and Europe to stay and build farms and communities. Many settlers were also part of a larger reverse migration of people who had originally settled in the Willamette Valley.
The Legislative Assembly created Gilliam County on February 25, 1885, from the eastern third of Wasco County after residents complained that they were too far from the county seat in The Dalles. The first county seat was at Alkali, now Arlington. The question of a permanent county seat was placed on general election ballots in 1886, 1888, and again in 1890, when voters chose to move the county seat to Condon, known to early settlers as "Summit Springs." Once the question of the location of the county seat was settled, voters in Gilliam County proved reluctant to provide a courthouse in Condon. The county government operated out of a two-room house until 1903, when the county court appropriated money to construct a courthouse. This courthouse burned down in 1954 and was replaced the following year with the current courthouse.