Rosalia is a town in Whitman County, Washington, United States. It is an agricultural community in the Palouse region, at an elevation of 2,238 feet above sea level. Its population was 550 at the 2010 ce…Rosalia is a town in Whitman County, Washington, United States. It is an agricultural community in the Palouse region, at an elevation of 2,238 feet above sea level. Its population was 550 at the 2010 census.