Clay Township is a civil township of St. Clair County in the U.S. state of Michigan. The population was 9,066 at the 2010 Census. The township is located along the mouth of the St. Clair River at Lake St. Clair, and the river delta includes Harsens Island and numerous other islands. The township mostly surrounds the city of Algonac, but the two are administered autonomously since Algonac incorporated as a city in 1967.