Cottage Grove is a city in Lane County, Oregon, United States. Its population was 10,643 at the 2020 census. It is the third largest city i…Cottage Grove is a city in Lane County, Oregon, United States. Its population was 10,643 at the 2020 census. It is the third largest city in Lane County. It is on Interstate 5, Oregon Route 99, and the main Willamette Valley line of the CORP railroad.