Joseph Sill Clark, Jr. (October 21, 1901January 12, 1990) was an American lawyer and politician. A member of the Democratic Party, he served as a United States Senator from Pennsylvania from 1957 to 1969. He previously served as the 90th Mayor of Philadelphia from 1952 to 1956. Clark was the only Unitarian Universalist elected to a major office in Pennsylvania in the modern era.