Montesano is a city in Grays Harbor County, Washington, United States. The population was 3,976 at the 2010 census. It is the county seat of Grays Harbor County.
Montesano was first settled in 1852 by Isaiah L. Scammon.
According to Edmond S. Meany, in 1860 the county seat of what was then called Chehalis County was moved to "the place of J.L. Scammons". Mrs. Lorinda Scammon was very religious and wished to have the place named Mount Zion. Another early settler, Samuel James, suggested "Montesano" as having a more pleasant sound and meaning about the same. The suggestion was approved and soon after a post office was secured with the name Montesano.