Harrisburg is a city in Linn County, Oregon, United States. The population was 3,567 at the 2010 census. According to the United States Censu…Harrisburg is a city in Linn County, Oregon, United States. The population was 3,567 at the 2010 census. According to the United States Census Bureau, the city has a total area of 1.45 square miles, of which 1.40 square miles is land and 0.05 square miles is water.