Old Greenwich is a coastal village in Fairfield County, Connecticut, United States. As of the 2010 census it had a population of 6,611. The town of Greenwich is one political and taxing body, but consists of several distinct sections or neighborhoods, such as Byram, Cos Cob, Glenville, Mianus, Old Greenwich, Riverside, and Greenwich. Of these neighborhoods, three have separate postal names and ZIP codes.