Alsea is an unincorporated community in Benton County, in the U.S. state of Oregon. It is on Oregon Route 34 and the Alsea River. For statistical purposes, the United States Census Bureau has defined Alsea as a census-designated place. The census definition of the area may not precisely correspond to the local understanding of the area with the s…Alsea is an unincorporated community in Benton County, in the U.S. state of Oregon. It is on Oregon Route 34 and the Alsea River. For statistical purposes, the United States Census Bureau has defined Alsea as a census-designated place. The census definition of the area may not precisely correspond to the local understanding of the area with the same name. As of the 2023 Census population estiamtes, the population was 220.