Philomath is a city in Benton County, Oregon, United States. It was named for Philomath College. The population was 5,838 as of the 2023 cens…Philomath is a city in Benton County, Oregon, United States. It was named for Philomath College. The population was 5,838 as of the 2023 census population estimates. It is part of the Corvallis, Oregon, Metropolitan Statistical Area.