Sans Souci is an unincorporated community in St. Clair County in the U.S. state of Michigan. The community is located within Clay Township. The community is located along the St. Clair River on the eastern coast of Harsens Island. The community itself is also referred to as Harsens Island, which is the name of the post office that serves the island and several smaller outlying islands near the river mouth on Lake St. Clair.