Plymouth is an unincorporated community in Benton County, Washington, United States. Plymouth is located on the Columbia River across from Umatilla, Oregon. It is south of the Tri-Cities and is served by Interstate 82. It was named by early settlers who felt a nearby rock promontory resembled Plymouth Rock.

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  Plymouth, Washington as seen from the Oregon side of the Columbia River at the mouth of the Umatilla River.
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  Plymouth, Washington post office as seen in July 2013.