Plymouth is a town in Plymouth County, Massachusetts, United States. It is the largest municipality in Massachusetts by area.The population was 51,701 at the 2000 census, with an estimated 2009 population of 58,681.Plymouth is one of two county seats of Plymouth County, the other being Brockton.It is named after Plymouth, Devon, United Kingdom, which is, in turn, named after its location at mouth of the River Plym.