Calais is a city in Washington County, Maine, United States. As of the 2020 census, it had a population of 3,079, making Calais the largest …Calais is a city in Washington County, Maine, United States. As of the 2020 census, it had a population of 3,079, making Calais the largest municipality by population in Washington County, but the third least-populous city in Maine. The city has three Canada–US border crossings over the St. Croix River connecting to St. Stephen, New Brunswick, Canada.