The Tualatin Plains are a prairie area in central Washington County, Oregon, United States.

Located around the Hillsboro and Forest Grove areas, the plains were first inhabited by the Atfalati band of the Kalapuya group of Native Americans.

[1] Congregational minister Harvey L. Clark started a missionary school in 1841 just north of East Tualatin Plains, now Hillsboro.

[2] The school was soon moved to West Tualatin Plains (now Forest Grove) where in 1847 Clark was joined by Tabitha Moffatt Brown, the Mother of Oregon.

[2] When the Provisional Government of Oregon formed in 1843, the Plains were part of the Twality District that was composed of the northwest section of the region, including what would become Portland.