Lacey is a city in Thurston County, Washington, United States. Established as a suburb of Olympia, Lacey's estimated population as of 2009 was 39,250, as compared to 43,900 for Olympia. Thurston County, which includes the cities of Lacey, Olympia, and Tumwater, has a population of 239,355 as of the 2009 census.